Serum thyroid hormone concentrations in clinically normal dogs after administration of freshly reconstituted vs previously frozen and stored thyrotropin

Concentrations of serum thyroxine (T4) and 3,3',5-triiodothyronine (T3) were determined in 7 clinically healthy adult dogs before and after administration of freshly reconstituted thyrotropin (TSH) and TSH that had been previously reconstituted and frozen for 1, 2, and 3 months. The 4 TSH response tests were performed at 30-day intervals by collecting blood samples for serum T4 and T3 determinations before and 4 and 6 hours after IV administration of TSH (0.1 U/kg of body weight). Baseline serum concentrations of T4 and T3 were similar at each of the 4 sample collection times over the 3-month period of the study. Mean serum concentrations of T4 and T3 increased significantly (P less than 0.01) over baseline values after administration of freshly reconstituted TSH or TSH that had been previously frozen for 1, 2, or 3 months. Significant difference was not found in the mean post-TSH serum T4 or T3 concentration after injection of freshly reconstituted TSH or TSH that had been previously frozen for 1, 2, or 3 months. In 2 of the 7 dogs, mild reactions--mild ataxia and weakness--were observed during the last of the series of TSH response tests (ie, after IV administration of TSH that had been previously frozen for 3 months). Results of this study suggest that for use in dogs, reconstituted TSH stored at -20 C maintains adequate biological activity for at least 3 months. The ability to store reconstituted TSH for a longer period than the recommended 48 hours represents an economic advantage, because it allows clinicians to perform more TSH response tests per vial of TSH.     

Effect of low doses of cosyntropin on serum cortisol concentrations in clinically normal dogs

OBJECTIVE: To determine the lowest of 5 doses of cosyntropin (1.0, 0.5, 0.1, 0.05, or 0.01 microg/kg) administered IV that stimulates maximal cortisol secretion in clinically normal dogs. ANIMALS: 10 clinically normal dogs. PROCEDURES: 5 dose-response experiments were performed in each of the dogs. Each dog received 5 doses of cosyntropin (1.0, 0.5, 0.1, 0.05, and 0.01 microg/kg) IV in random order (2-week interval between each dose). Serum samples for determination of cortisol concentrations were obtained before (baseline) and at 10, 20, 30, 40, 50, 60, 120, and 240 minutes after cosyntropin administration. RESULTS: Compared with baseline values, mean serum cortisol concentration in the study dogs increased significantly after administration of each of the 5 cosyntropin doses. Mean peak serum cortisol concentration was significantly lower after administration of 0.01, 0.05, and 0.1 microg of cosyntropin/kg, compared with findings after administration of 0.5 and 1.0 microg of cosyntropin/kg. After administration of 0.5 and 1.0 microg of cosyntropin/kg, mean peak serum cortisol concentration did not differ significantly; higher doses of cosyntropin resulted in more sustained increases in serum cortisol concentration, and peak response developed after a longer interval. CONCLUSIONS AND CLINICAL RELEVANCE: Administration of cosyntropin IV at a dose of 0.5 microg/kg induced maximal cortisol secretion in healthy dogs. Serum cortisol concentration was reliably increased in all dogs after the administration of each of the 5 doses of cosyntropin. These data should be useful in subsequent studies to evaluate the hypothalamic-pituitary-adrenal axis in healthy and critically ill dogs.     

Serum total thyroxine, total triiodothyronine, free thyroxine, and thyrotropin concentrations in epileptic dogs treated with anticonvulsants.

OBJECTIVE: To determine whether administration of phenobarbital, potassium bromide, or both drugs concurrently was associated with abnormalities in baseline serum total thyroxine (T4), triiodothyronine (T3), free T4, or thyrotropin (thyroid-stimulating hormone; TSH) concentrations in epileptic dogs. DESIGN: Prospective case series. ANIMALS: 78 dogs with seizure disorders that did not have any evidence of a thyroid disorder (55 treated with phenobarbital alone, 15 treated with phenobarbital and bromide, and 8 treated with bromide alone) and 150 clinically normal dogs that were not receiving any medication. PROCEDURE: Serum total T4, total T3, free T4, and TSH concentrations, as well as serum concentrations of anticonvulsant drugs, were measured in the 78 dogs with seizure disorders. Reference ranges for hormone concentrations were established on the basis of results from the 150 clinically normal dogs. RESULTS: Total and free T4 concentrations were significantly lower in dogs receiving phenobarbital (alone or with bromide), compared with concentrations in clinically normal dogs. Administration of bromide alone was not associated with low total or free T4 concentration. Total T3 and TSH concentrations did not differ among groups of dogs. CLINICAL IMPLICATIONS: Results indicate that serum total and free T4 concentrations may be low (i.e., in the range typical for dogs with hypothyroidism) in dogs treated with phenobarbital. Serum total T3 and TSH concentrations were not changed significantly in association with phenobarbital administration. Bromide treatment was not associated with any significant change in these serum thyroid hormone concentrations.     

Effect of dosing and sampling time on serum thyroxine, free thyroxine, and thyrotropin concentrations in dogs following multidose etodolac administration

Thirty-eight dogs with orthopedic disorders received etodolac, an NSAID, at 10.0 to 13.3 mg/kg PO once daily for 14 to 19 days. Mean total thyroxine (T4), free thyroxine (fT4), and canine thyrotropin (cTSH) values before and after etodolac administration were compared using paired t-tests. A significant (P <.05) decrease in T4 values occurred after etodolac administration with 21% of these values falling below the reference range. A significant (P <.05) increase in cTSH following etodolac administration, but none of the values was above the reference range. No significant changes occurred in mean fT4 values; however, 10% of the values fell below the reference range. In conclusion, T4 and fT4 test results should be interpreted with caution in dogs receiving etodolac.     

Serum total thyroxine, total triiodothyronine, free thyroxine, and thyrotropin concentrations in dogs with nonthyroidal disease

OBJECTIVE: To determine whether nonthyroidal disease of various causes and severity is associated with abnormalities in baseline serum concentrations of total thyroxine (T4), triiodothyronine (T3), free T4, or thyrotropin (thyroid-stimulating hormone [TSH]) in dogs believed to be euthyroid. DESIGN: Case-control study. ANIMALS: 223 dogs with confirmed nonthyroidal diseases and presumptive normal thyroid function, and 150 clinically normal dogs. PROCEDURE: Serum total T4, total T3, free T4, and TSH concentrations were measured in dogs with confirmed nonthyroidal disease. Reference ranges for hormone concentrations were established on the basis of results from 150 clinically normal dogs. RESULTS: In dogs with nonthyroidal disease, median serum concentrations of total T4, total T3, and free T4 were significantly lower than those in clinically normal dogs. Median serum TSH concentration in sick dogs was significantly greater than that of clinically normal dogs. When stratified by severity of disease (ie, mild, moderate, and severe), dogs with severe disease had low serum concentrations of total T4, total T3, or free T4 more commonly than did dogs with mild disease. In contrast, serum TSH concentrations were more likely to remain within the reference range regardless of severity of disease. CONCLUSIONS AND CLINICAL RELEVANCE: Results indicate that serum total T4, free T4, and total T3 concentrations may be low (ie, in the hypothyroid range) in dogs with moderate to severe nonthyroidal disease. Serum TSH concentrations are more likely to remain within the reference range in sick dogs.     

Effects of phenobarbital treatment on serum thyroxine and thyroid-stimulating hormone concentrations in epileptic dogs.

OBJECTIVE: To determine whether phenobarbital treatment of epileptic dogs alters serum thyroxine (T4) and thyroid-stimulating hormone (TSH) concentrations. DESIGN: Cross-sectional study. ANIMALS: 78 epileptic dogs receiving phenobarbital (group 1) and 48 untreated epileptic dogs (group 2). PROCEDURE: Serum biochemical analyses, including T4 and TSH concentrations, were performed for all dogs. Additional in vitro analyses were performed on serum from healthy dogs to determine whether phenobarbital in serum interferes with T4 assays or alters free T4 (fT4) concentrations. RESULTS: Mean serum T4 concentration was significantly lower, and mean serum TSH concentration significantly higher, in dogs in group 1, compared with those in group 2. Thirty-one (40%) dogs in group 1 had serum T4 concentrations less than the reference range, compared with 4 (8%) dogs in group 2. All dogs in group 2 with low serum T4 concentrations had recently had seizure activity. Five (7%) dogs in group 1, but none of the dogs in group 2, had serum TSH concentrations greater than the reference range. Associations were not detected between serum T4 concentration and TSH concentration, age, phenobarbital dosage, duration of treatment, serum phenobarbital concentration, or degree of seizure control. Signs of overt hypothyroidism were not evident in dogs with low T4 concentrations. Addition of phenobarbital in vitro to serum did not affect determination of T4 concentration and only minimally affected fT4 concentration. CONCLUSIONS AND CLINICAL RELEVANCE: Clinicians should be aware of the potential for phenobarbital treatment to decrease serum T4 and increase TSH concentrations and should use caution when interpreting results of thyroid tests in dogs receiving phenobarbital.     

Serum concentrations of thyroxine and 3,5,3'-triiodothyronine before and after intravenous or intramuscular thyrotropin administration in dogs

Response to thyrotropin (TSH) was evaluated in 2 groups of mixed-breed dogs. Thyrotropin (5 IU) was administered IV to dogs in group 1 (n = 15) and IM to dogs in group 2 (n = 15). Venous blood samples were collected immediately before administration of TSH and at 2-hour intervals for 12 hours thereafter. In group 1, the maximum mean concentration (+/- SD) of thyroxine (T4; 7.76 +/- 2.60 micrograms/dl) and 3,5,3'-triiodothyroxine (T3; 1.56 +/- 0.51 ng/ml) was attained at postinjection hours (PIH) 8 and 6, respectively. However, the mean concentration of T4 at PIH 6 (7.21 +/- 2.39 micrograms/dl) was not different (P greater than 0.05) from the mean concentration at PIH 8. The maximum mean concentration of T4 (10.10 +/- 3.50 micrograms/dl) and T3 (2.22 +/- 1.24 ng/ml) in group 2 was attained at PIH 12 and 10, respectively. Because dogs given TSH by the IM route manifested pain during injection, had variable serum concentrations of T3 after TSH administration, and may require 5 IU to achieve maximal increases in serum T4 concentrations, IV administration of TSH is recommended. The optimal sampling time to observe maximal increases in T3 and T4 after IV administration of TSH was 6 hours. Repeat IV administration of TSH may cause anaphylaxis and, therefore, is not recommended.     

Serum-free thyroxine concentrations, measured by chemiluminescence assay before and after thyrotropin administration in healthy dogs, hypothyroid dogs, and euthyroid dogs with dermathopathies.

The purpose of this study was to determine the usefulness of free thyroxine (FT4) measured by chemiluminescence in evaluating thyroid function in dogs. Total thyroxine (TT4) concentration measured by radioimmunoassay (RIA) and FT4 measured by chemiluminescence were evaluated in 30 healthy dogs, 60 euthyroid dogs with concurrent dermatopathies, and 30 hypothyroid dogs before and after intravenous stimulation with 1 or 2 IU of thyrotropin (TSH). Median basal TT4 and median TT4 concentrations at 4 h post-TSH administration were not significantly different (P < 0.0001) between healthy dogs and euthyroid dogs with dermatopathies, but were significantly higher than those in hypothyroid dogs. In healthy dogs, the median TT4 concentrations at 4 and 6 h post-TSH administration were not significantly different. Median basal FT4 and median FT4 concentrations at 4 h post-TSH administration in healthy dogs were significantly lower (P < 0.0001) than those in euthyroid dogs with dermatopathies, but significantly higher than the same parameters in hypothyroid dogs. There was a significant difference between the median FT4 concentrations at 4 h post-TSH administration and median basal FT4 concentrations for healthy dogs and euthyroid dogs with dermatopathies, but not for hypothyroid dogs. Lastly, in healthy dogs, median FT4 concentrations at 4 and 6 h post-TSH administration were not significantly different. Free thyroxine measured by chemiluminescence was highly correlated (P < 0.0001; Spearman r = 0.91) with FT4 measured by the reference method for free hormone analysis, namely, equilibrium dialysis, when sera from 56 dogs were used.     

Effects of short-term oral administration of propranolol on tear secretion in clinically normal dogs

This study evaluated the effects of short-term oral administration of propranolol on tear secretion in 15 clinically normal crossbreed dogs. The treatment group (n = 8) received propranolol (2 mg/kg q8h) orally for 7 days. The control group (n = 7) received placebo during the study. Schirmer I tear tests were performed on both eyes 1 d prior to drug administration (T(0)), at 1 (T(1)), 3 (T(3)), and 7 (T(7)) days of treatment. Tear production in dogs, measured by STT, was not significantly reduced in both groups.     

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.     

Evaluation of plasma aldosterone concentrations before and after ACTH administration in clinically normal dogs and in dogs with various diseases

Plasma aldosterone concentrations were measured in response to adrenocorticotropic hormone (ACTH) gel administration in clinically normal dogs, in dogs with hypoadrenocorticism, and in dogs (with electrolyte abnormalities) that did not have hypoadrenocorticism. Baseline plasma aldosterone concentrations were determined from specimens obtained every 10 minutes for 3 hours from 2 dogs and every 30 minutes for 7.5 hours from 2 other dogs. During the evaluation period, plasma aldosterone concentrations varied by at least 50% in each dog. A randomized crossover design was used to compare changes in plasma aldosterone concentrations after administration of ACTH gel and physiologic NaCl solution. Dogs had significantly (P = 0.002) higher plasma aldosterone concentrations after administration of ACTH gel than after administration of NaCl solution. Plasma cortisol concentrations increased as expected after ACTH gel administration. Analysis of cortisol and aldosterone concentrations in the same specimens obtained at 7 sample collection times did not reveal significant linear correlation, and scatterplots did not indicate a nonlinear association. In addition, plasma aldosterone concentrations were determined in response to ACTH administration alone and to ACTH combined with a high dose of dexamethasone (0.1 mg/kg, IV). The plasma aldosterone response to ACTH alone was not significantly different from the response to ACTH combined with dexamethasone. For both tests, plasma aldosterone concentrations at 60 and 120 minutes after ACTH administration were significantly (P less than 0.0005 and P = 0.0001, respectively, increased, compared with base-line values. Six dogs with adrenocortical hypofunction, as determined by plasma cortisol concentrations before and after ACTH administration, had plasma aldosterone concentrations that were diminished or did not increase after ACTH administration.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Circadian variations of serum thyroxine, free thyroxine and 3,5,3'triiodothyronine concentrations in healthy dogs

This study was to determine the daily fluctuation of serum thyroxine (tT₄), free thyroxine (fT₄), 3,5,3''-triiodothyronine (T₃) concentrations in healthy dogs. Thyroid function of these dogs was evaluated on the basis of results of TSH response test. Samples for the measurement of serum tT₄, fT₄, and T₃ concentrations were obtained at 3-hour intervals from 8 : 00 to 20 : 00. Serum tT₄, fT₄, and T₃ concentrations were measured by the enzyme chemiluminescent immunoassay (ECLIA). Mean T₃ concentrations had no significant differences according to the sample collection time during the day. Mean tT₄ and fT₄ concentrations at 11 : 00 were 3.28 ± 0.86 µg/dl and 1.30 ± 0.37 ng/dl, respectively and mean tT₄ and fT₄ at 14:00 were 3.54 ± 1.15 µg/dl and 1.35 ± 0.12 ng/dl, respectively. These concentrations were significantly high compared with tT₄ and fT₄ concentrations at 8:00, which were 1.75 ± 0.75 µg/dl and 0.97 ± 0.25 ng/dl, respectively (p < 0.05). According to the sample collection time, mean tT₄ and fT₄ concentrations changed with similar fluctuation during the day. Based on these results, it was considered that measurement of serum tT₄ and fT₄ concentrations from 11 : 00 to 14 : 00 might more easily diagnose the canine hypothyroidism in practice.     

Effects of a mock ultrasonographic procedure on cortisol concentrations during low-dose dexamethasone suppression testing in clinically normal adult dogs

OBJECTIVE: To determine whether the stress of an ultrasonographic procedure would interfere with the suppressive effect of dexamethasone during a low-dose dexamethasone suppression test (LDDST) in healthy dogs. ANIMALS: 6 clinically normal adult dogs. PROCEDURE: In phase 1, an LDDST was performed 5 times at weekly intervals in each dog. Serum samples were obtained 0, 2, 4, 6, and 8 hours after dexamethasone injection. A mock 20-minute abdominal ultrasonographic examination was performed on all dogs at each time point during the LDDST on weeks 2 through 5. In phase 2, serum cortisol concentrations were measured before and immediately after a 20-minute mock abdominal ultrasonographic examination, as described for phase 1. RESULTS: We did not detect significant differences after dexamethasone injection when comparing median cortisol concentrations for weeks 2 to 5 (mock ultrasonographic procedure) with median concentration for week 1 (no mock ultrasonographic procedure). For 5 of the 6 dogs, cortisol concentrations after dexamethasone injection decreased to < 35.9 nmol/L after each mock ultrasonographic procedure and remained low for the duration of the LDDST. In phase 2, all dogs had significant increases in cortisol concentrations immediately after the mock ultrasonographic procedure. CONCLUSIONS AND CLINICAL RELEVANCE: A 20-minute mock abdominal ultrasonographic examination performed during LDDST did not alter results of the LDDST in most dogs. Cortisol concentrations measured immediately after a mock ultrasonographic examination were significantly increased. Ultrasonographic procedures should be performed a minimum of 2 hours before collection of samples that will be used to measure cortisol concentrations.     

Effects of chlorothiazide on urinary excretion of calcium in clinically normal dogs.

Administration of thiazide diuretics has been recommended to prevent calcium oxalate urolith development in dogs. To evaluate the effects of thiazide diuretics in dogs, 24-hour urine excretion of calcium was measured in 6 clinically normal Beagles after administration of chlorothiazide (CTZ) for 2 weeks, administration of CTZ for 10 weeks, and administration of calcium carbonate and CTZ for 2 weeks. Compared with baseline values, 24-hour urine calcium excretion did not decrease after CTZ administration. When CTZ was given at a high dosage (130 mg/kg of body weight), urinary calcium excretion was significantly (P < 0.04) higher than baseline values. Based on these observations, we do not recommend CTZ for treatment or prevention of canine calcium oxalate urolithiasis.     

Effect of sodium bicarbonate infusions on ionized calcium and total calcium concentrations in serum of clinically normal cats

The effects of sodium bicarbonate (0.5 mEq/kg of body weight, 1.0 mEq/kg, 2.0 mEq/kg, and 4.0 mEq/kg) on ionized and total calcium concentrations were determined in clinically normal cats. Also, serum pH, whole blood pH, and serum albumin, serum total protein, and serum phosphorus concentrations were measured. Intravenous administration of sodium bicarbonate to awake cats decreased serum ionized calcium and serum total calcium concentrations. All dosages of sodium bicarbonate were associated with significant decreases of serum ionized calcium concentration. This effect lasted for greater than 180 minutes when cats were given 2.0 mEq/kg or 4.0 mEq/kg. When cats were given 4 mEq of sodium bicarbonate/kg, serum ionized calcium concentration was significantly decreased, compared with that when cats were given lower doses, but only at 10 minutes after infusion. After sodium bicarbonate infusion, serum total calcium concentration, measured by ion-specific electrode and colorimetry, was lower than baseline values at most of the times evaluated. Decreases in serum ionized calcium and serum total calcium concentrations can be attributed only in part to an increase in serum or whole blood pH and to a decrease in serum protein concentration. Serum total calcium concentrations measured by ion-specific electrode and by colorimetry were positively correlated, but the variability was high. Only 44% of the variability in serum ionized calcium concentration could be predicted when serum total calcium, albumin, total protein, phosphorus, and bicarbonate concentrations and pH were considered.     

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)     

Detection of low serum immunoglobulin concentrations in clinically ill calves

The sensitivity, specificity, and accuracy of classification of 4 tests for failure of passive transfer (FPT) were examined in clinically ill neonatal calves. Comparisons were made with serum IgG1 concentrations determined by radial immunodiffusion. Serum samples were obtained from 27 clinically ill calves < or = 21 days of age. The results of 4 commonly used assays, the sodium sulfite turbidity test, the zinc sulfate turbidity test, refractometry, and the serum gamma-glutamyl transferase (GGT) activity test, were compared with radial immunodiffusion determinations of serum IgG1 concentration. Serum GGT activity using a 50 IU/L threshold resulted in correct classification of the highest percentage of calves (93%) with regard to their passive transfer status. The sodium sulfite test with a 1+ end point and refractometry using a 5.5 g/dL end point resulted in correct classification of 85% of the calves studied. When using the sodium sulfite test, the 2+ and 3+ test end points had lower specificity, 0.58 and 0.00, respectively, than the 1+ end point. This loss in specificity resulted in misclassification of calves with adequate serum immunoglobulin concentrations as having FPT. The zinc sulfate turbidity test was inadequately specific (0.33) and resulted in misclassification of 33% of calves.