PULSATILE THYROTROPIN SECRETION IN NONTHYROIDAL ILLNESS

Pulsatile and nocturnal TSH secretion was investigated in 16 healthy controls (group A) and 21 patients with nonthyroidal illness (NTI; group B). Ten patients had normal plasma T-3 and T-4 values (group B-1), and 11 patients had decreased plasma T-3 with normal or low plasma T-4 (group B-2). Mean 24-h TSH secretion in NTI patients was higher than that in controls (3.1 +/- 0.7 vs. 1.7 +/- 0.7 mU/L; P < 0.01; mean +/- SD). This was associated with an increased TSH pulse amplitude in NTI patients relative to controls [Desade, 0.79 +/- 0.46 vs. 0.44 +/- 0.20 mU/L (P < 0.01); Cluster, 0.64 +/- 0.42 vs. 0.39 +/- 0.20 mU/ L (P < 0.05)]. There was no difference in TSH pulse amplitude between subgroups Bl and By. TSH pulse frequency was not different between NTI patients and controls (similar to 10 pulses/24 h). The mean 24-h TSH concentration was significantly related to the mean 24-h TSH pulse amplitude but not to the mean 24-h TSH pulse frequency. The absolute nocturnal TSH surge in NTI patients was lower than that in controls (0.6 +/- 0.6 vs. 1.0 +/- 0.6 mU/L; P < 0.05). The lower nocturnal TSH surge in patients with decreased plasma T-3 (group B-2) was associated with a loss of the usual nocturnal increase nocturnal increase in TSH pulse frequency was preserved. This was also observed in the six patients (two in group B-1 and four in group B-2) in whom the nocturnal TSH surge was completely absent. The changes in nocturnal TSH secretion in NTI resemble those found in central hypothyroidism, suggesting that features of central hypothyroidism are involved in the euthyroid sick syndrome.