Pulsatile thyrotropin secretion in patients with Cushing's syndrome

Pulsatile and circadian thyrotropin (TSH) secretion were studied in 16 healthy controls and in three patients with Cushing's syndrome who were studied twice (before and after treatment). Blood was sampled every 10 minutes over 24 hours for TSH (immunoradiometric assay [IRMA]). Mean 24-hour TSH in Cushing's syndrome was lower than in controls (0.4 +/- 0.2 v 1.7 +/- 0.7 mU/L, P < .001), related to a lower mean 24-hour TSH pulse amplitude (Desade: 0.16 +/- 0.15 v 0.44 +/- 0.20 mU/L, P < .001; Cluster: 0.17 +/- 0.14 v 0.39 +/- 0.20 mU/L, P < .001; mean +/- SD). TSH pulse frequency was unchanged with approximately 10 pulses/24 h. The nocturnal TSH surge was diminished relative to controls (median-0, range- -0.03 to 0.2 mU/L v 0.9 and 0.3 to 2.5 mU/L, respectively), related to a loss of the usual nocturnal increase in TSH pulse amplitude, but not of TSH pulse frequency. In the eucorticoid state, there was an increase of mean 24-hour TSH to 1.3 +/- 0.8 mU/L (P < .001) under normalization of the mean 24-hour TSH pulse amplitude. The nocturnal TSH surge also increased (median, 0.21; range, 0.15 to 0.4 mU/L) under restoration of the usual nocturnal increase in TSH pulse amplitude. In conclusion, Cushing's syndrome is associated with a decrease of mean 24-hour plasma TSH, related to a decrease of TSH pulse amplitude (not of TSH pulse frequency). The nocturnal TSH surge is decreased in Cushing's syndrome associated with a loss of the usual nocturnal increase of TSH pulse amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)