Regulation of TSH secretion and thyroid function in Cushing's disease

In 15 untreated patients with Cushing's disease the regulation of TSH secretion and thyroid function was evaluated. The maximal increment of plasma TSH to TRH was 3.8 +/- 2.2 vs 8.7 +/- 2.9 muU/ml (patients with Cushing's disease vs controls; mean +/- SD; P less than 0.001). Free thyroxine-index was 21.8 +/- 5.2 vs 27.7 +/- 5.0 (P less than 0.001). Plasma T3 1.45 +/- 0.24 vs 1.96 +/- 0.32 nmol/l (P less than 0.001) and reverse T3 0.22 +/- 0.05 vs 0.20 +/- 0.08 nmol/l (N.S.). The maximal TSH increase in response to TRH was inversely correlated with plasma cortisol (P less than 0.05). The maximal increment of TSH to TRH and plasma T4, free T4 index and T3 are reduced in patients with Cushing's disease due to an impaired thyrotroph function. Plasma reverse T3, however, is normal which may be due to a simultaneous decrease in production and degradation.