Thyroid Axis in Patients With Cushing's Syndrome

Abstract

Thyroid hormone values and serum thyrotropin (thyroidstimulating hormone [TSH]) responses to the intravenous administration of 400 μg of protirelin were determined in ten patients with Cushing's syndrome and in ten matched normal subjects. In patients with Cushing's syndrome, the serum thyroxine (T₄) level was mildly depressed and free T₃ level was normal. The mean ( ± SD) concentrations of serum triiodothyronine (T₃) and free T₃ were both reduced in patients compared with normal subjects (P <.001). At 20 and 60 minutes after protirelin administration, serum TSH levels were, respectively, 3.3 ± 2.7 μU/mL and 2.6 ± 2.3 μU/mL in patients with Cushing's syndrome and 12.3 ± 5.4 μU/mL and 10.7 ± 5.4 μU/mL in normal subjects (P <.001). The reduced serum T₃ and free T₃ levels are due to a glucocorticoid suppressive effect on the peripheral conversion of T₃ to T₃. The protirelin test is of limited value in assessing the thyroid status because the response of TSH is frequently blunted or absent due to glucocorticoid excess. (Arch Intern Med 139:767-772, 1979)