Thyroid axis in patients with Cushing's syndrome.

Thyroid hormone values and serum thyrotropin (thyroid-stimulating hormone [TSH]) responses to the intravenous administration of 400 micrograms 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 T4 level was normal. The mean (+/- SD) concentrations of serum triiodothyronine (T3) and free T3 were both reduced in patients compared with normal subjects (P less than .001). At 20 and 60 minutes after protirelin administration, serum TSH levels were, respectively, 3.3 +/- 2.7 microU/mL and 2.6 +/- 2.3 microU/mL in patients with Cushing's syndrome and 12.3 +/- 5.4 microU/mL and 10.7 +/- 5.4 microU/mL in normal subjects (P less than .001). The reduced serum T3 and free T3 levels are due to a glucocorticoid suppressive effect on the peripheral conversion of T4 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.