Adenohypophyseal Function in Dogs with Primary Hypothyroidism and Nonthyroidal Illness

Background: A recent study of dogs with induced primary hypothyroidism (PH) demonstrated that thyroid hormone deficiency leads to loss of thyrotropin (TSH) hypersecretion, hypersomatotropism, hypoprolactinemia, and pituitary enlargement with large vacuolated "thyroid deficiency" cells that double-stained for growth hormone (GH) and TSH, indicative of transdifferentiation of somatotropes to thyrosomatropes.
Hypothesis: Similar functional changes in adenohypophyseal function occur in dogs with spontaneous PH as do in dogs with induced PH, but not in dogs with nonthyroidal illness (NTI).
Animals: Fourteen dogs with spontaneous PH and 13 dogs with NTI.
Methods: Adenohypophyseal function was investigated by combined intravenous administration of 4 hypophysiotropic releasing hormones (4RH test), followed by measurement of plasma concentrations of ACTH, GH, luteinizing hormone (LH), prolactin (PRL), and TSH. In the PH dogs this test was repeated after 4 and 12 weeks of thyroxine treatment.
Results: In 6 PH dogs, the basal TSH concentration was within the reference range. In the PH dogs, the TSH concentrations did not increase with the 4RH test. However, TSH concentrations increased significantly in the NTI dogs. Basal and stimulated GH and PRL concentrations indicated reversible hypersomatotropism and hyperprolactinemia in the PH dogs, but not in the NTI dogs. Basal and stimulated LH and ACTH concentrations did not differ between groups.
Conclusions and Clinical Importance: Dogs with spontaneous PH hypersecrete GH but have little or no TSH hypersecretion. Development of hyperprolactinemia (and possible galactorrhea) in dogs with PH seems to occur only in sexually intact bitches. In this group of dogs with NTI, basal and stimulated plasma adenohypophyseal hormone concentrations were not altered.     

Thyrotropin-releasing hormone-induced growth hormone secretion in dogs with primary hypothyroidism

Primary hypothyroidism in dogs is associated with increased release of growth hormone (GH). In search for an explanation we investigated the effect of intravenous administration of thyrotropin-releasing hormone (TRH, 10 microg/kg body weight) on GH release in 10 dogs with primary hypothyroidism and 6 healthy control dogs. The hypothyroid dogs had a medical history and physical changes compatible with hypothyroidism and were included in the study on the basis of the following criteria: plasma thyroxine concentration < 2 nmol/l and plasma thyrotropin (TSH) concentration > 1 microg/l. In addition, (99m)TcO(4)(-) uptake during thyroid scintigraphy was low or absent. TRH administration caused plasma TSH concentrations to rise significantly in the control dogs, but not in the hypothyroid dogs. In the dogs with primary hypothyroidism, the mean basal plasma GH concentration was relatively high (2.3+/-0.5 microg/l) and increased significantly (P=0.001) 10 and 20 min after injection of TRH (to 11.9+/-3.5 and 9.8+/-2.7 microg/l, respectively). In the control dogs, the mean basal plasma GH concentration was 1.3+/-0.1 microg/l and did not increase significantly after TRH administration. We conclude that, in contrast to healthy control dogs, primary hypothyroid dogs respond to TRH administration with a significant increase in the plasma GH concentration, possibly as a result of transdifferentiation of somatotropic pituitary cells to thyrosomatotropes.     

Dopaminergic-like activity in toxic fescue alters prolactin but not growth hormone or thyroid stimulating hormone in ewes

Studies were conducted to determine the specificity and cause of altered pituitary hormone secretion when ewes ingest endophyte-infected (Acremonium coenophialum) GI-307 tall fescue (toxic fescue). Plasma concentrations of prolactin (PRL) but not growth hormone (GH) or thyroid stimulating hormone (TSH) in ewes grazing toxic fescue were significantly lower (P < .01) than concentrations measured in ewes grazing orchardgrass (OG). Comparing hormone secretory responses of ewes grazing each grasstype, ewes on toxic fescue released less PRL following thyrotropin releasing hormone (TRH) challenge than ewes on OG. TSH responses to TRH were not affected by grasstype. At this dose of TRH, GH secretion was not significantly affected in either group of ewes. In a separate study, dopamine hydrochloride (DA) was infused into control ewes to define the effect of a pure dopamine agonist on basal and TRH-stimulated secretion of PRL, GH and TSH. DA depressed both basal and TRH-stimulated secretion of PRL without affecting the basal concentrations or responses of GH or TSH. Based on the assumption that the active agent in toxic fescue responsible for the observed hypoprolactinemia was a dopaminergic agonist, haloperidol (HAL), a DA receptor blocking drug, was administered to ewes grazing toxic fescue or OG. HAL evoked significant PRL secretion unaccompanied by any GH or TSH effect in both toxic fescue and OG ewes. Administration of HAL resulted in a gradual increase over 4 hr in PRL in toxic fescue ewes and prolonged the duration of the PRL response to TRH. No differences in circulating plasma concentrations of DA, epinephrine or norepinephrine were measured in ewes on troxic fescue or OG.

Alterations in pituitary hormone secretion due to toxic factors in fescue were confined to PRL. Hormone secretory responses to TRH and HAL suggest that the effects on PRL are mediated through dopamine-like activity in toxic fescue.     

Serum concentrations of thyroxine, 3,5,3'-triiodothyronine, thyrotropin, and prolactin in dogs before and after thyrotropin-releasing hormone administration

Serum concentrations of thyrotropin (TSH), prolactin, thyroxine, and 3,5,3'-triiodothyronine in 15 euthyroid dogs and 5 thyroidectomized and propylthiouracil-treated dogs after thyrotropin-releasing hormone (TRH) administration were measured. Although thyroidectomized and propylthiouracil-treated dogs had higher (P less than 0.01) base-line concentrations of TSH in serum than did euthyroid dogs, concentrations of TSH after TRH administration varied at 7.5, 15, and 30 minutes with 14 of 45 samples obtained from healthy dogs having lower TSH concentrations than before TRH challenge. Similarly, concentrations of 3,5,3'-triiodothyronine in the serum of euthyroid dogs 4 hours after TRH administration were similar (P less than 0.05) to concentrations before TRH challenge. Although the mean concentration of thyroxine in serum was elevated (P less than 0.05) 4 hours after administration of TRH to euthyroid animals, as compared with base-line levels, the individual response was variable with concentrations not changing or decreasing in 4 dogs. Therefore, the TRH challenge test as performed in the current investigation was of limited value in evaluating canine pituitary gland function. Although mean concentrations of TSH in serum were higher (P less than 0.05) in euthyroid dogs after TRH administration, the response was too variable among individual animals for accurate evaluation of pituitary gland function. Concentrations of prolactin in the sera of dogs after TRH administration, confirmed previous reports that exogenously administered TRH results in prolactin release from the canine pituitary and indicated that the TRH used was biologically potent.     

Combined pituitary hormone deficiency in german shepherd dogs with dwarfism

In German shepherd dogs pituitary dwarfism is known as an autosomal recessive inherited abnormality. To investigate whether the function of cells other than the somatotropes may also be impaired in this disease, the secretory capacity of the pituitary anterior lobe (AL) cells was studied by a combined pituitary AL stimulation test with four releasing hormones (4RH test) in four male and four female German shepherd dwarfs. In addition, the morphology of the pituitary was investigated by computed tomography. The physical features of the eight German shepherd dwarfs were primarily characterized by growth retardation and stagnant development of the hair coat. The results of the 4RH test confirmed the presence of hyposomatotropism. The basal plasma TSH and prolactin concentrations were also low and did not change upon stimulation. Basal plasma concentrations of LH were relatively low and responded only slightly to suprapituitary stimulation. With respect to the plasma FSH levels there was a clear gender difference. In the males plasma FSH concentrations remained below the detection limit throughout the 4RH test, whereas in the females the basal plasma FSH levels were slightly lower and there was only a small increase following suprapituitary stimulation, compared with the values in age-matched controls. In contrast, basal and stimulated plasma ACTH concentrations did not differ between the dwarfs and the controls. Computed tomography of the pituitary fossa revealed a normal sized pituitary with cysts in five dogs, an enlarged pituitary with cysts in two dogs, and a small pituitary gland without cysts in the remaining dog. The results of this study demonstrate that German shepherd dwarfs have a combined deficiency of GH, TSH, and prolactin together with impaired release of gonadotropins, whereas ACTH secretion is preserved. The combined pituitary hormone deficiency is associated with cyst formation and pituitary hypoplasia.     

Ghrelin-stimulation test in the diagnosis of canine pituitary dwarfism

This study investigated whether ghrelin, a potent releaser of growth hormone (GH) secretion, is a valuable tool in the diagnosis of canine pituitary dwarfism. The effect of intravenous administration of ghrelin on the release of GH and other adenohypophyseal hormones was investigated in German shepherd dogs with congenital combined pituitary hormone deficiency and in healthy Beagles. Analysis of the maximal increment (i.e. difference between pre- and maximal post-ghrelin plasma hormone concentration) indicated that the GH response was significantly lower in the dwarf dogs compared with the healthy dogs. In none of the pituitary dwarfs, the ghrelin-induced plasma GH concentration exceeded 5 microg/l at any time. However, this was also true for 3 healthy dogs. In all dogs, ghrelin administration did not affect the plasma concentrations of ACTH, cortisol, TSH, LH and PRL . Thus, while a ghrelin-induced plasma GH concentration above 5 microg/l excludes GH deficiency, false-negative results may occur.     

The chicken pituitary-specific transcription factor PIT-1 is involved in the hypothalamic regulation of pituitary hormones

Pit-1 is a pituitary-specific POU-domain DNA binding factor, which binds to and trans-activates promoters of growth hormone- (GH), prolactin- (PRL) and thyroid stimulating hormone-beta- (TSHbeta) encoding genes. Thyrotropin-releasing hormone (TRH) is located in the hypothalamus and stimulates TSH, GH and PRL release from the pituitary gland. In the present study, we successfully used the cell aggregate culture system for chicken pituitary cells to study the effect of TRH administration on the ggPit-l* (chicken Pit-1), GH and TSHbeta mRNA expression in vitro. In pituitary cell aggregates of 11-day-old male broiler chicks the ggPit-l * mRNA expression was significantly increased following TRH administration, indicating that the stimulatory effects of TRH on several pituitary hormones are mediated via its effect on the ggPit-l* gene expression. Therefore, a semiquantitative RT-PCR method was used to detect possible changes in GH and TSHbeta mRNA levels. TRH affected both the GH and TSHbeta mRNA levels. The results of this in vitro study reveal that ggPit-1 * has a role in mediating the stimulatory effects of TRH on pituitary hormones like GH and TSHbeta in the chicken pituitary.     

Assessment of thyroid function in dogs with low plasma thyroxine concentration

BACKGROUND: Differentiation between hypothyroidism and nonthyroidal illness in dogs poses specific problems, because plasma total thyroxine (TT4) concentrations are often low in nonthyroidal illness, and plasma thyroid stimulating hormone (TSH) concentrations are frequently not high in primary hypothyroidism. HYPOTHESIS: The serum concentrations of the common basal biochemical variables (TT4, freeT4 [fT4], and TSH) overlap between dogs with hypothyroidism and dogs with nonthyroidal illness, but, with stimulation tests and quantitative measurement of thyroidal 99mTcO4(-) uptake, differentiation will be possible. ANIMALS: In 30 dogs with low plasma TT4 concentration, the final diagnosis was based upon histopathologic examination of thyroid tissue obtained by biopsy. Fourteen dogs had primary hypothyroidism, and 13 dogs had nonthyroidal illness. Two dogs had secondary hypothyroidism, and 1 dog had metastatic thyroid cancer. METHODS: The diagnostic value was assessed for (1) plasma concentrations of TT4, fT4, and TSH; (2) TSH-stimulation test; (3) plasma TSH concentration after stimulation with TSH-releasing hormone (TRH); (4) occurrence of thyroglobulin antibodies (TgAbs); and (5) thyroidal 99mTcO4(-) uptake. RESULTS: Plasma concentrations of TT4, fT4, TSH, and the hormone pairs TT4/TSH and fT4/TSH overlapped in the 2 groups, whereas, with TgAbs, there was 1 false-negative result. Results of the TSH- and TRH-stimulation tests did not meet earlier established diagnostic criteria, overlapped, or both. With a quantitative measurement of thyroidal 99mTcO4(-) uptake, there was no overlap between dogs with primary hypothyroidism and dogs with nonthyroidal illness. CONCLUSIONS AND CLINICAL IMPORTANCE: The results of this study confirm earlier observations that, in dogs, accurate biochemical diagnosis of primary hypothyroidism poses specific problems. Previous studies, in which the TSH-stimulation test was used as the "gold standard" for the diagnosis of hypothyroidism may have suffered from misclassification. Quantitative measurement of thyroidal 99mTcO- uptake has the highest discriminatory power with regard to the differentiation between primary hypothyroidism and nonthyroidal illness.     

Effects of growth hormone secretagogues on the release of adenohypophyseal hormones in young and old healthy dogs

The effects of three growth hormone secretagogues (GHSs), ghrelin, growth hormone-releasing peptide-6 (GHRP-6), and growth hormone-releasing hormone (GHRH), on the release of adenohypophyseal hormones, growth hormone (GH), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinising hormone (LH), prolactin (PRL) and on cortisol were investigated in young and old healthy Beagle dogs. Ghrelin proved to be the most potent GHS in young dogs, whereas in old dogs GHRH administration was associated with the highest plasma GH concentrations. The mean plasma GH response after administration of ghrelin was significantly lower in the old dogs compared with the young dogs. The mean plasma GH concentration after GHRH and GHRP-6 administration was lower in the old dogs compared with the young dogs, but this difference did not reach statistical significance. In both age groups, the GHSs were specific for GH release as they did not cause significant elevations in the plasma concentrations of ACTH, cortisol, TSH, LH, and PRL. It is concluded that in young dogs, ghrelin is a more powerful stimulator of GH release than either GHRH or GHRP-6. Ageing is associated with a decrease in GH-releasing capacity of ghrelin, whereas this decline is considerably lower for GHRH or GHRP-6.     

Thyroid-stimulating hormone responses after single administration of thyrotropin-releasing hormone and combined administration of four hypothalamic releasing hormones in beagle dogs

Thyrotropin (TSH) responses were determined in eight healthy male beagle dogs after a single administration of thyrotropin-releasing hormone (TRH) and the combined administration of four hypothalamic releasing hormones, i.e., corticotropin-releasing hormone, growth hormone-releasing hormone, gonadotropin-releasing hormone, and TRH. In both tests, TRH was administered in a dose of 10 μg/kg. Basal TSH concentrations ranged form 0.07 to 0.27 μg/1(mean ± SE, 0.14 ± 0.02 μg/1). The administration of TRH, alone or in the combined test, resulted in a prompt and significant increase in TSH with mean (±SE) plasma TSH peaks of 1.26 ± 0.22 μg/1 at 10 min and 0.85 ± 0.17 μg/1 at 30 min, respectively. The area under the curve (0–120 min) was significantly lower in the combined test than in the single TRH test, whereas the increments were not significantly different. It is concluded that measurements of TSH responses to TRH alone and in combination with other releasing hormones can be used for the assessment of pituitary thyrotropic cell function. In the combined test, the TSH response is slightly lower than that in the single test.     

Regulation of growth hormone expression by thyrotropin-releasing hormone through the pituitary-specific transcription factor Pit-1 in chicken pituitary

Pit-1 is a pituitary-specific POU-domain DNA binding factor, which binds to and trans-activates promoters of growth hormone- (GH), prolactin- (PRL) and thyroid stimulating hormone beta- (TSHbeta) encoding genes. Pit-1 has been identified in several mammalian and avian species. Thyrotropin-releasing hormone (TRH) is located in the hypothalamus and it stimulates TSH, GH and PRL release from the pituitary gland. In the present study, we successfully developed a competitive RT-PCR for the detection of Pit-1 expression in the chicken pituitary, that was sensitive enough to detect picogram levels of Pit-1 mRNA. Applying this method, the effect of TRH injections on Pit-1 mRNA expression was determined in the pituitary of chick embryos and growing chicks. In both 18-day-old embryos and 10-day-old male chicks the Pit-1 mRNA expression was significantly increased following TRH injection, thereby indicating that the stimulatory effects of TRH on several pituitary hormones is mediated via its effect on Pit-1 expression. Therefore, a semi-quantitative RT-PCR method was used to detect possible changes in GH levels. TRH affected the GH mRNA levels at both developmental stages. These results, combined with the data on Pit-1 mRNA expression, indicate that Pit-1 has a role in mediating the stimulatory effects of TRH on pituitary hormones like GH.     

Somatotroph and lactotroph function in relation to growth in six-week-old pigs reared in a hot or cool environment

The influences of thermal environment and individual growth rate on somatotroph and lactotroph function were examined in 6-week-old barrows reared entirely in a hot (H: 27–32°C, n = 8) or cool (C: 21°C, n = 10) environment. Growth hormone (GH) and prolactin (PRL) cell contents and responses to growth hormone-releasing hormone (GHRH) or thyrotropin-releasing hormone (TRH) were evaluated in cultured pituitary cells from each animal. Plasma GH, PRL, and insulin-like growth factor-1 (IGF-1) concentrations also were monitored. Thermal environment did not affect in vitro GH secretion, cellular GH content, or plasma GH concentrations. Stimulated in vitro GH release (GHRH-basal) and plasma GH were inversely related to average daily gain (ADG, r = −.76, p < .005 and r = −.51, p < .05, respectively). Cellular GH content also declined as ADG increased (r = −.57, p < .05). Plasma IGF-1 concentrations were not affected by thermal environment and were not related to ADG. Pituitary cells from H animals secreted and contained more PRL than cells from C animals (p < .05). Plasma PRL values were correlated with ADG (r = .54, p < .05), but did not differ between thermal groups. Stimulated in vitro PRL (TRH-vehicle) secretion was positively related with ADG only in the H group (r = .97, p < .001). In contrast, cellular PRL content decreased with ADG in cells from the H barrows (r = −.8, p < .05). Lactotroph function was not related to growth in cells from C pigs. In summary, 1) heat enhanced PRL secretion and cell content; 2) growth and somatotroph function were inversely related; and 3) serum PRL and the PRL response to TRH in cells from H barrows were positively related to growth.     

Effects of thyrotropin-releasing hormone on serum concentrations of thyroxine and triiodothyronine in healthy, thyroidectomized, thyroxine-treated, and propylthiouracil-treated dogs

Effects of thyrotropin-releasing hormone (TRH) on serum concentrations of thyroid hormones were studied in 36 mixed-bred dogs. Dogs were randomly assigned to 7 groups. Significant increases (P less than 0.05) of serum thyroxine (T4) values occurred as early as 2 hours and reached a peak at 6 to 8 hours after IV injection of 300 to 1,100 micrograms of TRH. Thyroxine concentrations in response to a TRH dose greater than 500 micrograms were similar to those observed with the 300-micrograms dose. Transient coughing, vomiting, salivation, and defecation after large doses (900 and 1,100 micrograms) were observed. Mean serum T4 concentration decreased from 2.1 micrograms/dl to 0.9 micrograms/dl within 1 day of thyroidectomy. Clinical signs of hypothyroidism, including lethargy, dry coats, and diffuse alopecia, were present in 2 dogs at a month after surgical operation. Thyroxine concentrations were detectable for greater than 2 months. Injection (IV) of 700 micrograms of TRH 6 weeks after surgical operation had no effect on serum concentration of T4 in thyroidectomized dogs. In 5 T4-treated dogs, TRH (700 micrograms, IV) significantly increased the serum T4 value, indicating that pituitary thyrotropes were responsive to TRH, in spite of daily medication of 0.8 mg of T4. Four dogs were treated orally with 200 mg of propylthiouracil/day for 5 weeks. Intravenous injection of 700 micrograms of TRH in propylthiouracil-treated dogs had no effect on the serum T4 concentration, indicating that TRH had no effect on serum T4 values in these dogs during the experimental period. These results indicate that TRH can replace bovine thyrotropin for the canine thyroid function test.(ABSTRACT TRUNCATED AT 250 WORDS)     

Suppression of somatotroph function induced by growth hormone treatment in neonatal pigs

The effect of recombinant porcine growth hormone (pGH) treatment on pituitary function was evaluated in young pigs. Piglets received intraperitoneal recombinant pGH implants (0.5 mg/d sustained release) or vehicle implants beginning at 3 d of age. Ten piglets were sacrificed at 4 and 6 wk of age (five piglets/treatment group) for the collection of pituitary glands, blood, and liver tissue. Blood samples also were drawn at 3 and 12 d of age. Serum concentrations of GH, prolactin (PRL), thyroid-stimulating hormone (TSH), insulin-like growth factor-1 (IGF-1) and IGF-2 were evaluated. Levels of IGF-1 and IGF-2 mRNA were determined in liver samples. Treatment with GH increased circulating levels of GH and IGF-1 (P < 0.01), but not PRL, TSH, or IGF-2. Hepatic IGF-1, but not IGF-2, mRNA levels were increased by pGH (P < 0.001). Cultured pituitary cells from each animal were challenged with 0.1, 1, and 10 nM GH-releasing hormone (GHRH); 2 mM 8-Br-cAMP; or 100 nM phorbol myristate acetate. The release of GH from cultured pituitary cells was stimulated by all secretagogues (P < 0.001). The secretion of GH, but not PRL or TSH, in culture was inhibited by previous in vivo GH treatment (P < 0.001). Similarly, cellular GH, but not PRL or TSH, content was lower in the GH-implant group (P = 0.005). Cell cultures from 6-wk-old piglets secreted more GH, but not PRL or TSH, than cultures from 4-wk-old piglets (P < 0.05). Likewise, cellular GH, but not PRL or TSH, content was greatest in cultures from 6-wk-old animals (P = 0.002). Piglet growth was not affected by exogenous GH treatment (P = 0.67). These results demonstrate that exogenous pGH treatment selectively down-regulates somatotroph function in young pigs.     

Endocrine adaptations of periparturient swine to alteration of dietary energy source

Lipid compared with carbohydrate calories in the diet of peripartal sows was examined for specific metabolic effects. Blood samples from second-parity Yorkshire sows fed isocaloric-isonitrogenous diets that differed by either glucose or corn oil supplying 20% of the daily metabolizable energy intake were assayed for growth hormone (GH), insulin and prolactin (PRL) concentrations. On d 97 to 103 of gestation, serum GH levels were reduced following feed intake; the reduction was greater (P less than .05) in glucose-fed sows during the postprandial period. Insulin concentration was increased following consumption of either diet, but to a greater (P less than .05) extent in glucose-fed sows corresponding to the greater (P less than .05) blood glucose response. Basal, fasting concentrations of PRL were not affected by dietary energy source; however, thyrotropin-releasing hormone (TRH) infusion resulted in a greater (P less than .05) PRL response of fat-fed sows. Samples obtained on d 8 to 11 of lactation revealed that TRH stimulated (P less than .05) GH secretion and the response was greater (P less than .05) in fat-fed sows. Plasma insulin concentration was increased (P less than .05) following feeding, and the increase was greater (P less than .05) in glucose-fed sows. Similar to the response measured in gestating sows, TRH-induced increases in PRL was greater in fat-fed, lactating sows. Sham-nursing did not influence maternal PRL concentration compared with the immediate increase noted following natural nursing. No effect of dietary energy source on reproductive performance characteristics was noted. Results of this study have shown that dietary lipid, compared with carbohydrate, selectively increased the sensitivity of the pituitary gland to a GH and PRL secretagogue such as TRH. The beneficial effects of lipid in the diet of peripartal swine may be attributable, in part, to the effect of dietary lipid on maternal pituitary PRL synthesis and(or) release.     

Concentrations of Prolactin, LH, Testosterone, TSH and Thyroxine in Normospermic Dogs of Different Breeds

Concentrations of prolactin (PRL), LH, testosterone (T), TSH and thyroxine (T₄) were determined before and at 20, 120 and 180 min after a single iv injection of thyrotropin-releasing hormone (TRH) in eight Beagles, eight Fox Terriers, six Labrador Retrievers and five Great Danes that were normospermic. Mean basal PRL concentrations were lower in the Fox Terriers compared with the Great Danes (p < 0.05). Mean LH concentrations were higher in the Fox Terriers than in the Beagles, and T was lower in the Fox Terriers at some times but not others (p < 0.05). Thyroid Stimulating Hormone (TSH) concentrations did not differ among breeds, while mean basal T₄ values were lower in Fox Terriers compared with Labrador Retrievers and Great Danes (p < 0.05). Stimulation of T₄ secretion 120 and 180 min after iv TRH injection was most pronounced in the Beagles and less in the Fox Terriers (p < 0.05). The results of the present study indicate that potential breed differences in circulating concentrations of PRL, LH, T, TSH and T₄ in male dogs with apparently normal fertility can be encountered, but further studies are needed to determine whether the observed differences are typical features of these breeds, reflect subsets of dogs within breeds, or are in part because of possible uncontrolled parameters such as sample timing, ambient photoperiod, housing conditions or diet.     

Tertiary hypothyroidism in a dog

: A nine-year-old male entire Labrador was diagnosed with pituitary dependent hyperadrenocorticism. Following seven months of successful mitotane therapy, the dog presented with marked weight gain, seborrhoea and alopecia. Routine clinicopathological analyses revealed marked hypercholesterolaemia. Serum total and free thyroxine (T4) concentrations were below their respective reference ranges. Serum thyroid stimulating hormone (cTSH) concentration was within reference range. TSH and thyrotropin releasing hormone (TRH) response tests revealed adequate stimulation of total T4 in both, and cTSH in the latter test. Magnetic resonance imaging revealed a mass arising from the pituitary fossa, with suprasellar extension. A diagnosis of tertiary hypothyroidism was made. Following four weeks of levothyroxine therapy, circulating cholesterol concentration had declined, weight loss had ensued and dermatological abnormalities had improved. Euthanasia was performed four months later due to the development of neurological signs. A highly infiltrative pituitary adenoma, with effacement of the overlying hypothalamus was identified on post mortem examination. Tertiary hypothyroidism has not been previously reported in dogs.     

The drop in plasma thyrotropin concentrations in fasted chickens is caused by an action at the level of the hypothalamus: role of corticosterone

Fasting has severe effects on thyroid metabolism in the chicken: plasma thyroxine (T₄) concentrations increase, whereas 3′,5,3-triiodothyronine (T₃) concentrations decrease. In the present report we studied the effect of fasting at the level of: 1) the pituitary (plasma thyrotropin (TSH) concentrations; the sensitivity of thyrotrophs to corticotropin-releasing hormone (CRH) and TSH-releasing hormone (TRH)); and 2) the hypothalamus (TRH content). A regulatory role of corticosterone is discussed. One day of fasting resulted in a drop in plasma TSH concentrations. Fed and nonfed animals were treated with ovine CRH (oCRH) or TRH. The sensitivity of thyrotrophs to the respective hypothalamic hormones was increased when animals were subjected to a 1-d period of fasting. A 75% (TRH) and 50% (oCRH) increase in plasma TSH was recorded in fasted animals, whereas both secretagogues did not evoke any response in their fed counterparts. The drop in plasma TSH cannot, therefore, be attributed to a loss in sensitivity of thyrotrophs to hypothalamic stimulatory control. In an identical experiment, plasma TSH concentrations decreased, whereas hypothalamic TRH content was higher in fasted animals, suggesting a decreased hypothalamic TRH release toward the pituitary. In both fasting experiments, plasma corticosterone concentrations were increased after 1 d of fasting. Because an iv injection of corticosterone-elevated hypothalamic TRH contents and decreased plasma TSH concentrations, a corticosterone-induced TSH decrease during fasting is suggested through an action at the level of the hypothalamus.     

Thyroid function testing in Greyhounds.

OBJECTIVE: To evaluate thyroid function in healthy Greyhounds, compared with healthy non-Greyhound pet dogs, and to establish appropriate reference range values for Greyhounds. ANIMALS: 98 clinically normal Greyhounds and 19 clinically normal non-Greyhounds. PROCEDURES: Greyhounds were in 2 groups as follows: those receiving testosterone for estrus suppression (T-group Greyhounds) and those not receiving estrus suppressive medication (NT-group Greyhounds). Serum thyroxine (T4) and free thyroxine (fT4) concentrations were determined before and after administration of thyroid-stimulating hormone (TSH) and thyroid-releasing hormone (TRH). Basal serum canine thyroid stimulating hormone (cTSH) concentrations were determined on available stored sera. RESULTS: Basal serum T4 and fT4 concentrations were significantly lower in Greyhounds than in non-Greyhounds. Serum T4 concentrations after TSH and TRH administration were significantly lower in Greyhounds than in non-Greyhounds. Serum fT4 concentrations after TSH and TRH administration were significantly lower in NT-group than T-group Greyhounds and non-Greyhounds. Mean cTSH concentrations were not different between Greyhounds and non-Greyhounds. CONCLUSIONS AND CLINICAL RELEVANCE: Previously established canine reference range values for basal serum T4 and fT4 may not be appropriate for use in Greyhounds. Greyhound-specific reference range values for basal serum T4 and fT4 concentrations should be applied when evaluating thyroid function in Greyhounds. Basal cTSH concentrations in Greyhounds are similar to non-Greyhound pet dogs.     

Relationships between the thyroid and somatotropic axes in steers I: Effects of propylthiouracil-induced hypothyroidism on growth hormone, thyroid stimulating hormone and insulin-like growth factor I

The effects of propylthiouracil (PTU)-induced thyroid hormone imbalance on GH, TSH and IGF-I status in cattle were examined. In the first study, four crossbred steers (avg wt 350 kg) were fed a diet dressed with PTU (0, 1, 2 or 4 mg/kg/d BW) in a Latin square design with four 35-d periods. On day 29 in each period, steers were challenged with an intrajugular bolus of thyrotropin releasing hormone (TRH, 1.0 μg/kg). Blood samples were obtained to assess the change in plasma GH and TSH as affected by PTU. Plasma IGF-I was measured from blood samples obtained before and after (every 6 hr for 24 hr) intramuscular injection of bovine GH (0.1 mg/kg, day 31). Doses of 1 and 2 mg/kg PTU increased plasma T₄ (P<.01). At 4 mg/kg, PTU depressed T₄ concentrations to 30% of control (P<.01). Plasma T₃ linearly decreased with increasing doses of PTU (P<.01). Plasma TSH increased when PTU was fed at 4 mg/kg (P<.05) while the TSH response to TRH declined with increasing PTU (P<.02). Neither basal nor TRH-stimulated plasma concentration of GH was affected by PTU; the IGF-I response to GH tended to increase at the 1 and 2 mg/kg PTU (P<.01). In a second study 24 crossbred steers were fed PTU (1.5 mg/kg) for 119 d in a 2 × 2 factorial design with implantation of the steroid growth effector, Synovex-S (200 mg progesterone + 20 mg estradiol), as the other main effect. Basal plasma GH and IGF-I were not affected by PTU treatment. Synovex increased plasma concentration (P<.01) of IGF-I without an effect on plasma GH. The data suggest that mild changes in thyroid status associated with PTU affects regulation of T₃, T₄ and TSH more than GH or IGF-I in steers.