Assessment of dose and time responses to TRH and thyrotropin in healthy dogs

Changes in total thyroxine (T₄ [TT₄]), free T₄(FT₄) and total tri-iodothyronine (T₃ [TT₃]) in serum after the intravenous administration of different doses of thyrotropin (TSH) and thy-rotropin-releasing hormone (TRH) were measured in six healthy beagles. Significant (P<0·05) elevations in serum TT₄, FT₄ and TT₃ were observed at each sampling time (two, four, five, six, seven, eight and 10 hours) after administration of 1, 3 or 5 iu (total dose) TSH and peak mean responses were observed six to eight hours after injection. At six hours after injection the mean TT₄, FT₄ and TT₃ levels were approximately 2·6, 3·9 and 1·5 times basal levels, respectively, and there were no significant differences between the three doses of TSH. Significant (P<0·05) elevations in serum TT₄ and FT₄ but not TT₃ were observed at each sampling time (two, four, five, six, seven and eight hours) after the administration of TRH. Peak mean responses were observed at four hours after injection at which time TT₄ and FT₄ levels were approximately 1·7 and 1·9 times basal levels, respectively. No significant differences were observed between the four doses of TRH used (100, 200, 300 and 600 μg total dose). Concentrations of TT₄, FT₄ and TT₃ were significantly (P<0·05) higher following the administration of TSH compared with TRH, and the response to TRH showed greater individual variation.     

Thyroid-stimulating hormone in adult euthyroid and hypothyroid horses

The purpose of this study was to validate a thyroid-stimulating hormone (TSH) assay in a model of equine hypothyroidism. Thyrotropin-releasing hormone (TRH) stimulation tests were performed in 12 healthy adult mares and geldings, aged 4 to greater than 20 years. before and during administration of the antithyroid drug propylthiouracil (PTU) for 6 weeks. Serum concentrations of equine TSH, total and free thyroxine (T4), and total and free triiodothyronine (T3) were measured. Before PTU administration, mean +/- standard deviation baseline concentrations of TSH were 0.40 +/- 0.29 ng/mL. TSH increased in response to TRH, reaching a peak concentration of 0.78 +/- 0.28 ng/mL at 45 minutes. Total and free T4 increased from 12.9 +/- 5.6 nmol/L and 12.2 +/- 3.5 pmol/L to 36.8 +/- 11.4 nmol/L and 23.1 +/- 5.9 pmol/L, respectively, peaking at 4-6 hours. Total and free T3 increased from 0.99 +/- 0.51 nmol/L and 2.07 +/- 1.14 pmol/L to 2.23 +/- 0.60 nmol/l and 5.78 +/- 1.94 pmol/L, respectively, peaking at 2-4 hours. Weekly measurements of baseline TSH and thyroid hormones during PTU administration showed that total and free T, concentrations fell abruptly and remained low throughout PTU administration. Total and free T4 concentrations did not decrease dramatically until weeks 5 and 4 of PTU administration, respectively. A steady increase in TSH concentration occurred throughout PTU administration, with TSH becoming markedly increased by weeks 5 and 6 (1.46 +/- 0.94 ng/mL at 6 weeks). During weeks 5 and 6 of PTU administration, TSH response to TRH was exaggerated, and thyroid hormone response was blunted. Results of this study show that measurement of equine TSH in conjunction with thyroid hormone measurement differentiated normal and hypothyroid horses in this model of equine hypothyroidism.     

Serum concentrations of thyroxine and 3,5,3'-triiodothyronine in dogs before and after administration of freshly reconstituted or previously frozen thyrotropin-releasing hormone

Concentrations of serum thyroxine (T4) and 3,5,3'-triiodothyronine (T3) were determined after the administration of freshly reconstituted thyrotropin-releasing hormone (TRH), reconstituted TRH that had been previously frozen, or thyrotropin (TSH) to 10 mature dogs (6 Greyhounds and 4 mixed-breed dogs). Thyrotropin-releasing hormone (0.1 mg/kg) or TSH (5 U/dog) was administered IV; venous blood samples were collected before and 6 hours after administration of TRH or TSH. Concentrations of the T4 and T3 were similar (P greater than 0.05) in serum after administration of freshly reconstituted or previously frozen TRH, indicating that TRH can be frozen at -20 C for at least 1 week without a loss in potency. Concentrations of T4, but not T3, were higher after the administration of TSH than they were after the administration of TRH (P less than 0.01). Concentrations of T4 increased at least 3-fold in all 10 dogs given TSH, whereas a 3-fold increase occurred in 7 of 10 dogs given freshly reconstituted or previously frozen TRH. Concentrations of T4 did not double in 1 dog given freshly reconstituted TRH and in 1 dog given previously frozen TRH. Concentrations of T3 doubled in 5 of 10, 2 of 10, and 5 of 10 dogs given TSH, freshly reconstituted TRH, or previously frozen TRH, respectively. Results suggested that concentrations of serum T4 are higher 6 hours after the administration of TSH than after administration of TRH, using dosage regimens of 5 U of TSH/dog or 0.1 mg of TRH/kg.(ABSTRACT TRUNCATED AT 250 WORDS)     

Equine thyroid function assessment with the thyrotropin-releasing hormone response test

The effect of thyrotropin-releasing hormone (TRH) on equine thyroid function was determined by quantifying serum thyroxine (T4) and 3,5,3'-triiodothyronine (T3) before and after TRH administration. Thyrotropin-releasing hormone was administered IV to adult horses (n = 5) and ponies (n = 6) at a dose of 1 mg or 0.5 mg, respectively. Serum T4 and T3 concentrations were determined before and 0.25, 0.5, 1, 2, 4, 6, 8, 12, and 24 hours after TRH administration. Serum T4 increased from a basal concentration of 24.4 +/- 8.7 ng/ml (mean +/- SD) to a maximum value of 48.2 +/- 10.2 by 4 hours after TRH administration. Serum T3 increased from a basal concentration of 0.44 +/- 0.18 ng/ml to a maximum value of 1.31 +/- 0.37 ng/ml by 2 hours after TRH administration. Seemingly, TRH increases serum concentrations of T4 and T3 and may be useful as a test of equine hypophysis-thyroid function.     

Equine thyroid function tests: a preliminary investigation.

A similar and significant (P less than 0.001) increase in plasma thyroxine (T4) concentration was seen in seven clinically normal thoroughbred horses 2 h after the intravenous administration of either 2.5 iu or 5 iu of thyroid stimulating hormone (TSH) with a peak response around 4 h after administration. The intravenous administration of 0.2, 0.5 or 1 mg thyrotrophin releasing hormone (TRH) resulted in a significant (P less than 0.01) increase in T4 concentration in three groups of animals; six thoroughbreds in full work, five thoroughbreds at rest and four ponies at rest. The peak response was recorded at 3 or 4 h after administration. A significant difference between the groups in the degree of response to TRH was only found between the thoroughbreds in work and those at rest with 1 mg TRH (P less than 0.05). When two additional ponies were investigated in a similar way, a reduced response to TRH was observed: a pregnant mare had a similar response to 5 iu TSH as the thoroughbreds; the other pony also showed a lowered response to TSH. In a group of 2- or 3-year-old thoroughbreds in training no difference in the T4 response 4 h after intravenous administration of 0.5 mg TRH could be determined, according to the month, age, sex or work intensity. Although resting T4 concentrations did not differ significantly between animals believed to be suffering from the equine rhabdomyolysis syndrome (ERS) and those suffering from a variety of other conditions, some ERS sufferers may have a lowered response to TRH.     

Effects of thyrotropin-releasing hormone and a thyrotropin releasing hormone analog on growth and selected plasma hormones in lambs

An 8-wk growth trial was conducted to assess the effects of continuous infusion of thyrotropin-releasing hormone (TRH) and an active TRH analog less than Aad-His-Pro-NH2 (the less than Aad is L-pyro-alpha-aminoadipic acid) on growth trial performance, carcass composition and hormone profiles of growing lambs. Both drugs were infused at 600 micrograms X lamb -1 X d -1 with 16 lambs/treatment. Both TRH and less than Aad-His-Pro-NH2 decreased average daily gain (ADG; P less than .01) and increased feed conversion (FC; P less than .01) compared with saline infused controls. Average daily feed intake was not altered. Carcasses of lambs given TRH or less than Aad-His-Pro-NH2 contained fewer kilograms of moisture (P less than .05) and appeared to contain fewer kilograms of protein. Thyrotropin-releasing hormone and less than Aad-His-Pro-NH2 increased thyroid gland weights (P less than .05), but pituitary gland weights were not different. Plasma thyrotropin (TSH) concentrations were increased by both drugs compared with control lambs, peaking at 4 to 7 d after initiating infusion. However, by 14 d, TSH concentrations returned to control levels. Triiodothyronine (T3) and thyroxine (T4) were elevated by both drugs over the entire 8-wk trial, with peak levels reached at 10 d and maintained for the duration of the study. Both TRH and less than Aad-His-Pro-NH2 increased prolactin over the entire period. Growth hormone levels were not altered by either drug. The effects of less than Aad-His-Pro-NH2 infusion on growth trial performance, carcass composition and hormone profiles of growing lambs were very similar to TRH. The negative effects of TRH and less than Aad-His-Pro-NH2 infusion on ADG, FC and carcass protein appear to be the result of elevated T3 and T4 levels.     

Assessment of thyroid functional reserve in the cat by the thyrotropin-stimulation test

Serum thyroxine (T4) concentrations before and after various IV doses of bovine thyrotropin (TSH) were measured over a 48-hour period in 19 healthy cats. Base-line T4 values, as measured by radioimmunoassay, varied greatly. The peak T4 concentration occurred 6 hours after TSH injection, and there was an increase in post-TSH serum T4 concentration that was linearly related to the logarithm of the dose. Greatest stimulation was seen with the highest dose used (1 U of TSH/kg of body weight), and 6 hours after administration of this dose, the serum T4 concentration range was 4.1 to 8.4 micrograms/dl. The post-TSH serum T4 concentration and the absolute increase in serum T4 concentration after TSH administration correlated more closely with the TSH dose than did the ratio of post-TSH serum T4 concentration to base-line T4 concentration. Therefore, in cats with normal thyroid-binding protein concentrations, the former indices should represent the most reliable assessment of thyroid functional reserve.     

Effect of thyrotropin releasing hormone (TRH) on serum levels of thyroid hormones in thoroughbred mares

Effects of thyrotropin releasing hormone (TRH) on serum levels of thyroid hormones were studied in 12 Thoroughbred mares. Significant increases (P<0.05) of serum T4 levels occurred as early as 2 hours and peaked at 4–10 hours after intravenous injection of 0.5 – 5 mg TRH. Following injection of 0.5, 1, 3 and 5 mg of TRH the serum levels of T4 were increased 2.25, 2.42,2.42 and 3.67 fold, respectively, over pre-injection levels. Serum levels of T3 were also significantly increased (P<0.05) at 1 or 2 hours and peaked at 2 to 4 hoursafter injection. The mean peak increase of T₃ levels were 2.87, 3.21, 3.10 and 3.10 fold over pre- injected in level in 0.5, 1, 3,5 mg treated horses, respectively. These results suggest that TRH can be an alternative to heterologous TSH for the equine thyroid function test. The recommended dosage is 1–3 mg, and most appropriate time to collect post-TRH blood sample is between 4–6 hours. Serum levels of T₄ and T₃ should increase 2–3 fold from baseline in normal horses.     

冷应激对雏鸡下丘脑-垂体-甲状腺轴的影响

越来越多的证据表明,应激能够激活下丘脑-垂体-甲状腺轴,进而影响促甲状腺激素释放激素(TRH)、促甲状腺激素(TSH)及甲状腺激素的合成与分泌。为了探明冷应激对雏鸡下丘脑-垂体-甲状腺轴的影响,以公雏鸡为实验动物,进行急性(0.25、1、3、6、12h与24h)与慢性(5、10d与20d)冷应激处理(12±1)℃,检测了雏鸡下丘脑TRH mRNA的表达水平,血清TSH、FT(3T3的游离形式)及FT(4T4的游离形式)的含量。结果表明:急性应激时,TRH mRNA的表达水平在各应激时间点均显著升高,TSH变化不明显,FT3开始无变化,在6h时突然降低,而后又显著升高,FT4开始变化不大,6h后显著升高;慢性应激时,TRH mRNA的表达水平与相应的对照组相比显著降低,TSH变化仍不明显,FT3呈上升趋势,而FT4呈下降趋势。这说明冷暴露可以使雏鸡下丘脑-垂体-甲状腺轴的激素分泌发生改变,而且不同程度的冷应激对同一激素也会产生不同的影响。     

Use of recombinant human thyroid-stimulating hormone for thyrotropin-stimulation testing of euthyroid cats

OBJECTIVE: To evaluate response of euthyroid cats to administration of recombinant human thyroid-stimulating hormone (rhTSH). ANIMALS: 7 healthy cats. PROCEDURE: Each cat received each of 5 doses of rhTSH (0, 0.025, 0.050, 0.100, and 0.200 mg), IV, at 1-week intervals. Serum concentration of total thyroxine (TT4) and free thyroxine (fT4) was measured immediately before each injection (time 0) and 2, 4, 6, and 8 hours after administration of each dose. RESULTS: Overall TT4 response did not differ significantly among cats when administered doses were > or = 0.025 mg. Serum TT4 concentrations peaked 6 to 8 hours after administration for all doses > or = 0.025 mg. For all doses > or = 0.025 mg, mean +/- SEM TT4 concentration at 0, 6, and 8 hours was 33.9 +/- 1.7, 101.8 +/- 5.9, and 101.5 +/- 5.7 nmol/L, respectively. For all doses > or = 0.025 mg, mean fT4 concentration at 0, 6, and 8 hours was 38.7 +/- 2.9, 104.5 +/- 7.6, and 100.4 +/- 8.0 pmol/L, respectively. At 8 hours, the fT4 response to 0.025 and 0.050 mg was less than the response to 0.100 and 0.200 mg. Adverse reactions after rhTSH administration were not detected. CONCLUSIONS AND CLINICAL RELEVANCE: The TSH stimulation test can be performed in cats by IV administration of 0.025 to 0.200 mg of rhTSH and measurement of serum TT4 concentrations at time of injection and 6 or 8 hours later. Clinical validation of the TSH stimulation test would facilitate development of additional tests of thyroid gland function, such as a TSH assay.     

Effects of oral administration of levothyroxine sodium on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone in healthy adult mares

OBJECTIVE: To determine the effects of levothyroxine sodium (L-T4) on serum concentrations of thyroid gland hormones and responses to injections of thyrotropin-releasing hormone (TRH) in euthyroid horses. ANIMALS: 12 healthy adult mares. PROCEDURE: 8 horses received an incrementally increasing dosage of L-T4 (24, 48, 72, or 96 mg of L-T4/d) for weeks 1 to 8. Each dose was provided for 2 weeks. Four additional horses remained untreated. Serum concentrations of total triiodothyronine (tT3), total thyroxine (tT4), free T3 (fT3), free T4 (fT4), and thyroid-stimulating hormone (TSH) were measured in samples obtained at weeks 0, 2, 4, 6, and 8; 1.2 mg of TRH was then administered i.v., and serum concentrations of thyroid gland hormones were measured 2 and 4 hours after injection. Serum reverseT3 (rT3) concentration was also measured in the samples collected at weeks 0 and 8. RESULTS: Treated horses lost a significant amount of weight (median, 19 kg). Significant treatment-by-time effects were detected for serum tT3, tT4, fT3, fT4, and TSH concentrations, and serum tT4 concentrations were positively correlated (r, 0.95) with time (and therefore dosage) in treated horses. Mean +/- SD serum rT3 concentration significantly increased in treated horses (3.06 +/- 0.51 nmol/L for week 8 vs 0.74 +/- 0.22 nmol/L for week 0). Serum tT3, tT4, fT3, and TSH concentrations in response to TRH injections differed significantly between treated and untreated horses. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of levothyroxine sodium increased serum tT4 concentrations and blunted responses toTRH injection in healthy euthyroid horses.     

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.     

Effect of recombinant bovine tumor necrosis factor-α on hormone release in lactating cows

Tumor necrosis factor (TNF)‐α is a powerful macrophage cytokine released during infection, circulating in the blood to produce diverse effects in the organism. We examined the effect of recombinant bovine TNF‐α (rbTNF‐α) administration on hormone release in dairy cows during early lactation. Twelve non‐pregnant Holstein cows were treated subcutaneously with rbTNF‐α (2.5 µg/kg) or saline twice (at 11.00 and 23.00 hours). At 11.00 hours the next day, the cows were given growth hormone‐releasing hormone (GHRH, 0.25 µg/kg), thyrotrophin‐releasing hormone (TRH, 1.0 µg/kg), thyroid‐stimulating hormone (TSH, 10 µg/kg) or adrenocorticotropic hormone (500 µg/head) via the jugular vein. In the growth hormone‐releasing hormone challenge, the plasma growth hormone concentration was lower in the rbTNF‐α group than in the control (saline) group. The growth hormone and TSH responses to TRH were also smaller in the rbTNF‐α group than in the control. The plasma prolactin response to TRH was not affected by the rbTNF‐α treatment. In the TSH challenge, the rbTNF‐α‐treated cows had lower responses, as measured by plasma triiodothyronine and thyroxine, than the control cows. The rbTNF‐α treatment produced an increase in the basal plasma cortisol level, but the cortisol response to adrenocorticotropic hormone was the same level in both groups. The plasma concentrations of TNF‐α and interleukin‐1β in the cows were elevated by the rbTNF‐α treatment. The milk yield was reduced by the rbTNF‐α administration during 4 days. These data demonstrate that TNF‐α alters the secretion of pituitary and thyroid hormones in lactating cows. This effect may contribute to the suppression of the lactogenic function of the mammary gland observed in cases of coliform mastitis with high circulating TNF‐α levels.     

Effects of oral chlortetracycline and dietary protein level on plasma concentrations of growth hormone and thyroid hormones in beef steers before and after challenge with a combination of thyrotropin-releasing hormone and growth hormone-releasing hormone.

The objective of this study was to determine the effect of a subtherapeutic level of chlortetracycline (CTC) fed to growing beef steers under conditions of limited and adequate dietary protein on plasma concentrations of GH, thyroid-stimulating hormone (TSH), and thyroid hormones before and after an injection of thyrotropin-releasing hormone (TRH) + GHRH. Young beef steers (n = 32; average BW = 285 kg) were assigned to a 2x2 factorial arrangement of treatments of either a 10 or 13% crude protein diet (70% concentrate, 15% wheat straw, and 15% cottonseed hulls) and either a corn meal carrier or carrier + 350 mg of CTC daily top dressed on the diet. Steers were fed ad libitum amounts of diet for 56 d, and a jugular catheter was then placed in each steer in four groups (two steers from each treatment combination per group) during four consecutive days (one group per day). Each steer was injected via the jugular catheter with 1.0 microg/kg BW TRH + .1 microg/kg BW GHRH in 10 mL of saline at 0800. Blood samples were collected at -30, -15, 0, 5, 10, 15, 20, 30, 45, 60, 120, 240, and 360 min after releasing hormone injection. Plasma samples were analyzed for GH, TSH, thyroxine (T4), and triiodothyronine (T3). After 84 d on trial, the steers were slaughtered and the pituitary and samples of liver were collected and analyzed for 5'-deiodinase activity. Feeding CTC attenuated the GH response to releasing hormone challenge by 26% for both area under the response curve (P<.03) and peak response (P<.10). Likewise, CTC attenuated the TSH response to releasing hormone challenge for area under the response curve by 16% (P<.10) and peak response by 33% (P<.02), and attenuated the T4 response for area under the curve by 12% (P<.08) and peak response by 14% (P<.04). Type II deiodinase activity in the pituitary was 36% less (P<.02) in CTC-fed steers than in steers not fed CTC. The results of this study are interpreted to suggest that feeding subtherapeutic levels of CTC to young growing beef cattle attenuates the release of GH and TSH in response to pituitary releasing hormones, suggesting a mechanism by which CTC may influence tissue deposition in cattle.     

Thyroid and adrenal function tests in adult male ferrets

Effects of thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH) on plasma concentrations of thyroid hormones, and effects of ACTH and dexamethasone on plasma concentrations of cortisol, were studied in adult male ferrets. Thirteen ferrets were randomly assigned to test or control groups of eight and five animals, respectively. Combined (test + control groups) mean basal plasma thyroxine (T4) values were different between the TRH (1.81 +/- 0.41 micrograms/dl, mean +/- SD) and TSH (2.69 +/- 0.87 micrograms/dl) experiments, which were performed 2 months apart. Plasma T4 values significantly (P less than 0.05) increased as early as 2 hours (3.37 +/- 1.10 micrograms/dl) and remained high until 6 hours (3.45 +/- 0.86 micrograms/dl) after IV injection of 1 IU of TSH/ferret. In contrast, IV injection of 500 micrograms of TRH/ferret did not induce a significant increase until 6 hours (2.75 +/- 0.79) after injection, and induced side effects of hyperventilation, salivation, vomiting, and sedation. There was no significant increase in triiodothyronine (T3) values following TSH or TRH administration. Combined mean basal plasma cortisol values were not significantly different between ACTH stimulation (1.29 +/- 0.84 micrograms/dl) and dexamethasone suppression test (0.74 +/- 0.56 micrograms/dl) experiments. Intravenous injection of 0.5 IU of ACTH/ferret induced a significant increase in plasma cortisol concentrations by 30 minutes (5.26 +/- 1.21 micrograms/dl), which persisted until 60 minutes (5.17 +/- 1.99 micrograms/dl) after injection. Plasma cortisol values significantly decreased as early as 1 hour (0.41 +/- 0.13 micrograms/dl), and had further decreased by 5 hours (0.26 +/- 0.15 micrograms/dl) following IV injection of 0.2 mg of dexamethasone/ferret.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thyroid stimulating hormone and prolactin secretion after thyrotropin releasing hormone administration to mares: Dose response during anestrus in winter and during estrus in summer

The response of thyroid stimulating hormone (TSH) and prolactin (PRL) concentrations to administration of thyrotropin releasing hormone (TRH) was determined in light-horse mares during the anestrous season (winter) and during estrus (standing heat) in the summer. Within each season, mares (4/group) were treated with either saline (controls) or one of four doses of TRH (80, 400, 2,000 or 10,000 ug) intravenously. Samples of blood were drawn at −15, −.5, 15, 30, 45, 60, 90, 120, 180 and 240 min relative to TRH injection. Concentrations of TSH and PRL in pre-TRH samples were greater (P<.05) in anestrous mares during winter than in estrous mares during summer. Concentrations of TSH increased (P<.05) within 30 min after administration of TRH and remained elevated during the 4-hr sampling period. The maximal net change in TSH concentrations and the area under the response curve were greatest for 2,000 ug of TRH; 80 ug did not produce a significant TSH response. There was no interaction (P >.10) between reproductive state and TRH dose for TSH concentrations. Concentrations of PRL were not significantly affected by any TRH dose during either season. It appears that mares differ from many mammalian species in that they do not respond to an injection of TRH with increases in both TSH and PRL.     

Radioimmunoassay of serum total thyroxine and triiodothyronine in healthy cats: assay methodology and effects of age, sex, breed, heredity and environment.

Double antibody radioimmunoassays for thyroxine (T4) and triiodothyronine (T3) were established for use in cats, using standards in T4-and T3-free, normal cat serum. The assay working ranges were 5–200 nmol/1 for T4 and 0.125-10 nmol/1 for T3. Recovery of added hormone was 106 per cent ± 5.9 for T4 and 104 per cent ± 8.5 for T3 (mean ± 1 s.d.). Mean between-assay coefficients of variation (c.v.) were 6.6 per cent for T4 and 12–1 per cent for T3. Serum total T4 levels of 318 and total T3 levels of 299 healthy cats aged 4 months to 13 years were measured. Mean ± 1 s.d. values were 26.1 ± 10.1 nmol/1 for T4 and 0.69 ± 0.29 nmol/1 for T3. T4 and T3 concentrations within individual animals were highly correlated. T4 levels in both sexes tended to decrease until approximately 5 years of age and then rise again. A similar, though less pronounced effect, was found for T3, concentrations for older cats levelling out rather than rising. For any given age, females and neutered females tended to have significantly higher T4 values than males and neutered males, but such effects were not significant for T3. When other effects were accounted for, the effects of neutering were not significant for levels of either hormone. Pedigree animals tended to have higher levels of T3 at any given age than domestic short- and long-haired cats taken as one group. Animals living in the same environment had significantly similar T4 levels and T3 levels. For T3, this appeared to be due to a definite genetic component, but it was not possible to differentiate between environmental and genetic effects for T4.     

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.     

Evaluation of serum free thyroxine and thyrotropin concentrations in the diagnosis of canine hypothyroidism

Canine thyroid-stimulating hormone (cTSH), total thyroxine (T4) and free T4 by equilibrium dialysis (fT4d) were measured in serum samples from 107 dogs with clinical signs suggestive of hypothyroidism in which the diagnosis was either confirmed (n = 30) or excluded (n = 77) by exogenous TSH response testing. Median serum total T4 and fT4d concentrations were significantly lower and cTSH significantly higher (P < 0.001) in hypothyroid compared with euthyroid dogs. Differential positive rate analysis determined optimal cut-off values of less than 14.9 nmol/litre (total T4), less than 5.42 pmol/litre (fT4d), greater than 0.68 ng/ml (cTSH), less than 17.3 (T4 to cTSH ratio), and less than 7.5 (fT4d to cTSH ratio) for hypothyroidism. These had a sensitivity and specificity of 100 and 75.3 per cent, 80 and 93.5 per cent, 86.7 and 81.8 per cent, 86.7 and 92.2 per cent, and 80 and 97.4 per cent, respectively, for diagnosing hypothyroidism. Corresponding areas under the receiver operating characteristic curves were 0.92, 0.93, 0.87, 0.93 and 0.93. Unexpectedly low cTSH values in hypothyroid dogs may have resulted from concurrent non-thyroidal illness. Unexpectedly high serum cTSH values in the euthyroid dogs might have resulted from recovery from illness or concurrent potentiated sulphonamide therapy. Measurement of endogenous cTSH concentration is a valuable diagnostic tool for canine hypothyroidism if used in association with assessment of T4. Estimation of fT4d added only limited additional information over total T4 measurement.     

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.