Objective: To determine whether measurement of baseline serum concentrations of total thyroxine (T₄), triiodothyronine (T₃), free T₄, and thyrotropin (thyroid-stimulating hormone; TSH) would aid in the diagnosis of hypothyroidism in dogs. Design: Prospective case series. Animals: 54 dogs with hypothyroidism, 54 euthyroid dogs with nonthyroidal disease initially suspected to have hypothyroidism, and 150 clinically normal dogs. Procedure: In the 54 dogs with hypothyroidism, diagnosis was established on the basis of clinical signs, results of routine laboratory and TSH stimulation tests, exclusion of concurrent nonthyroidal disease, and a good clinical response to treatment with l-thyroxine. Blood samples were collected from all dogs and were tested for thyroid hormone and TSH concentrations. Reference ranges for hormone concentrations were established on the basis of results for the 150 clinically normal dogs. Results: Of the 54 hypothyroid dogs, 48 (89%) had low total T₄ concentrations, 3 had low-normal concentrations, and 3 had high concentrations because of T₄ autoantibodies. In contrast, only 10 (18%) euthyroid dogs had low total T₄ concentrations. Only 3 of 31 (10%) hypothyroid dogs had low T₃ concentrations; 23 had concentrations within the reference range, and 5 had high concentrations because of T₃ autoantibodies. Only 3 of 38 euthyroid dogs had low T₃ concentrations. Of the hypothyroid dogs, 53 (98%) had low free T₄ concentrations and 1 had a low-normal concentration. Only 4 (7%) euthyroid dogs had low free T₄ concentrations. Of the hypothyroid dogs, 41 (76%) had high TSH concentrations, and 13 had TSH concentrations within the reference range. Of the euthyroid dogs, only 4 (8%) had high TSH concentrations. Of all single hormone measurements evaluated, measurement of free T₄ concentration had the highest sensitivity (0.98), specificity (0.93), and accuracy (0.95) as a test for hypothyroidism; measurement of total T₄ concentration had a lower sensitivity (0.89), specificity (0.82), and accuracy (0.85). Compared with measurement of total or free T₄ concentration, measurement of TSH concentration had a lower sensitivity (0.76) and accuracy (0.84) but specificity (0.93) equal to that for measurement of free T₄ concentration. When T₄ (total or free) and TSH concentrations were evaluated together, specificity was higher than when T₄ or TSH concentration was evaluated alone. Only 1 euthyroid dog had low T₄ (total and free) and high TSH concentrations. Clinical Implications: Results indicate that measurement of serum free T₄ and TSH concentrations is useful for diagnosis of hypothyroidism in dogs. About a quarter of the dogs with confirmed hypothyroidism, however, will have serum TSH concentrations within reference limits. (J Am Vet Med Assoc 1997;211:1396–1402)

Objective—To determine the usefulness of measuring serum free thyroxine (T4) concentration as a diagnostic test for hyperthyroidism in cats, and to determine the influence of nonthyroidal disease on free T₄ concentration in cats without hyperthyroidism. Design—Prospective case series. Animals—917 cats with untreated hyperthyroidism, 221 cats with nonthyroidal disease, and 172 clinically normal cats. Procedure—Serum free T₄, total T₄, and total triiodothyronine (T₃) concentrations were measured in cats with untreated hyperthyroidism and cats with nonthyroidal disease. Serum total T₄ and T₃ concentrations were determined by use of radioimmunoassay, and free T₄ concentration was measured by use of direct equilibrium dialysis. Reference ranges for hormone concentrations were established on the basis of results from the 172 clinically normal cats. Results—Sensitivity of serum free T₄ concentration as a diagnostic test for hyperthyroidism was significantly higher than the test sensitivity of either total T₄ or T₃ concentration. Of the 221 cats with nonthyroidal disease, 14 had a high free T₄ concentration (ie, falsepositive result). Therefore, calculated specificity of measuring serum free T₄ concentration as a diagnostic test for hyperthyroidism was significantly lower than test specificity of measuring either the total T₄ or T₃ concentration. Conclusions and Clinical Relevance—Results indicate that determination of free T₄ concentration is useful in the diagnosis of hyperthyroidism, especially in cats in which hyperthyroidism is suspected but total T₄ and T₃ concentrations are within reference ranges. However, because some cats with nonthyroidal disease have high serum free T₄ concentrations, hyperthyroidism should not be diagnosed solely on the finding of high free T₄ concentration. (J Am Vet Med Assoc 2001;218:529–536)

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. (J Am Vet Med Assoc 2001;219:765–769)

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 (ie, 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. (J Am Vet Med Assoc 1999;214:1804-1808)

Background: Treatment of adrenal-dependent hyperadrenocorticism (ADH) involves either surgical resection of the adrenal tumor or medical therapy. For many years, mitotane has been considered the medical treatment of choice for dogs with ADH. Objectives: The aim of this study was to determine survival and prognostic factors for dogs with ADH treated with mitotane and trilostane. Animals: Twenty-six dogs with ADH were included in the study. Methods: Fourteen dogs were treated with mitotane and 12 dogs were treated with trilostane. Medical records were reviewed. Epidemiologic factors, signalment, clinicopathologic abnormalities, endocrine test results, and treatment protocols were evaluated to identify potential predictive factors of overall survival time. Results: Survival times of dogs treated with mitotane (median, 15.6 months) or trilostane (median, 14.0 months) were not significantly different. Using univariate analysis, age and postadrenocorticotropic hormone cortisol concentrations were inversely correlated with survival time. The multivariate model also identified weakness at presentation as a negative prognostic indicator. Conclusion and Clinical Importance: The type of medical treatment (mitotane versus trilostane) does not influence survival time in dogs with ADH; therefore, trilostane, a drug with less frequent and milder adverse effects, might be used as the primary medical treatment when adrenalectomy cannot be performed.