Effects of spontaneous hyperadrenocorticism on serum thyroid hormone concentrations in the dog

Serum thyroid hormone concentrations were evaluated in 124 dogs with untreated spontaneous hyperadrenocorticism either by measuring basal thyroxine (T4) and triiodothyronine (T3) concentrations (102 dogs) or by assessing the T4 response to exogenous thyroid-stimulating hormone (TSH) administration (22 dogs). Reduced basal serum concentrations of T4 and of T3 were found in 58 (57%) and 53 (52%), respectively, of the 102 dogs with hyperadrenocorticism; of these, 42 dogs had low values for both T4 and T3, 16 had decreased T4 concentrations alone, and 11 had only decreased T3 concentrations. In 20 dogs that had basal serum thyroid hormone concentrations determined before and after control of hyperadrenocorticism, mean concentrations of both T4 and T3 increased significantly (P less than 0.05). Serum T4 and T3 concentrations normalized in all but 1 of the 20 dogs. In the 22 dogs with hyperadrenocorticism given TSH, mean serum concentrations of T4 at both basal and post-TSH administration times were significantly decreased (P less than 0.01) compared with the results obtained in 18 normal dogs. Nevertheless, a significant increase (P less than 0.001) in mean T4 concentration occurred in these dogs; T4 concentrations after TSH was given were at least 2-fold greater than basal values. The T4 response to exogenous TSH was less in the 13 dogs with low basal T4 concentrations than in the 9 dogs that had resting T4 values within the normal range.(ABSTRACT TRUNCATED AT 250 WORDS)