Serum free and total iodothyronine concentrations in dogs with hyperadrenocorticism

Duncan C. Ferguson From the Department of Physiology & Pharmacology and Department of Small Animal Medicine, The University of Georgia, College of Veterinary Medicine, Athens, GA 30602 (Ferguson), and Department of Medicine, The Animal Medical Center and Research Animal Resource Center, Cornell University Medical College, New York, NY 10021 (Peterson).

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Mark E. Peterson From the Department of Physiology & Pharmacology and Department of Small Animal Medicine, The University of Georgia, College of Veterinary Medicine, Athens, GA 30602 (Ferguson), and Department of Medicine, The Animal Medical Center and Research Animal Resource Center, Cornell University Medical College, New York, NY 10021 (Peterson).

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SUMMARY

Serum concentrations of total and free thyroxine (T4 and FT4, respectively), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (reverse T3) were measured in 42 dogs with hyperadrenocorticism, and were compared with values determined in clinically normal dogs. Mean total T4 concentration in dogs with hyperadrenocorticism (14.3 nmol/L) was significantly (P < 0.001) lower than the normal value (25.7 nmol/L), with 38% of the dogs having low serum T4 concentration. Although 16 (38%) of the 42 dogs with hyperadrenocorticism had a high FT4 fraction, indicative of diminished serum T4 binding, normal FT4 concentration was found in only 6 of the 16 dogs (38%) with low total T4 values. Mean serum T3 concentration in dogs with hyperadrenocorticism (0.79 nmol/L) was also significantly (P < 0.001) lower than the normal value (1.16 nmol/L), with 39% of the dogs having T3 values below the normal range. Individual T3-to-T4 and T3-to-FT4 ratios, indices of T3 production and/or clearance, were above the normal range in 29 and 24% of dogs with hyperadrenocorticism, respectively. Mean reverse T3 concentration in dogs with hyperadrenocorticism (0.17 nmol/L) was also significantly (P < 0.001) lower than the normal mean value (0.39 nmol/L), with 48% of the dogs having reverse T3 values below the normal range. Of the 21 dogs in which all iodothyronines were measured, 6 (29%) had undetectable values for all hormones. Therefore, of dogs with hyperadrenocorticism, a subgroup had reduced serum T4 binding and another group had low values for all iodothyronines consistent with secondary hypothyroidism, but few dogs had low T3-to-FT4 ratio (indicative of inhibition of T4 to T3 conversion) commonly seen in human patients with Cushing's syndrome. The pattern of changes in dogs was reminiscent of the changes observed in rats with chronic glucocorticoid excess, in which pituitary thyrotropin secretion is suppressed and T3 production and degradation are depressed. Despite the severity of the changes in FT4 concentration in some dogs, it is possible that overt hypothyroidism is averted by maintenance of serum T3 concentration.

SUMMARY

Serum concentrations of total and free thyroxine (T4 and FT4, respectively), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (reverse T3) were measured in 42 dogs with hyperadrenocorticism, and were compared with values determined in clinically normal dogs. Mean total T4 concentration in dogs with hyperadrenocorticism (14.3 nmol/L) was significantly (P < 0.001) lower than the normal value (25.7 nmol/L), with 38% of the dogs having low serum T4 concentration. Although 16 (38%) of the 42 dogs with hyperadrenocorticism had a high FT4 fraction, indicative of diminished serum T4 binding, normal FT4 concentration was found in only 6 of the 16 dogs (38%) with low total T4 values. Mean serum T3 concentration in dogs with hyperadrenocorticism (0.79 nmol/L) was also significantly (P < 0.001) lower than the normal value (1.16 nmol/L), with 39% of the dogs having T3 values below the normal range. Individual T3-to-T4 and T3-to-FT4 ratios, indices of T3 production and/or clearance, were above the normal range in 29 and 24% of dogs with hyperadrenocorticism, respectively. Mean reverse T3 concentration in dogs with hyperadrenocorticism (0.17 nmol/L) was also significantly (P < 0.001) lower than the normal mean value (0.39 nmol/L), with 48% of the dogs having reverse T3 values below the normal range. Of the 21 dogs in which all iodothyronines were measured, 6 (29%) had undetectable values for all hormones. Therefore, of dogs with hyperadrenocorticism, a subgroup had reduced serum T4 binding and another group had low values for all iodothyronines consistent with secondary hypothyroidism, but few dogs had low T3-to-FT4 ratio (indicative of inhibition of T4 to T3 conversion) commonly seen in human patients with Cushing's syndrome. The pattern of changes in dogs was reminiscent of the changes observed in rats with chronic glucocorticoid excess, in which pituitary thyrotropin secretion is suppressed and T3 production and degradation are depressed. Despite the severity of the changes in FT4 concentration in some dogs, it is possible that overt hypothyroidism is averted by maintenance of serum T3 concentration.

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