PULSATILE THYROTROPIN SECRETION IN NONTHYROIDAL ILLNESS

Pulsatile and nocturnal TSH secretion was investigated in 16 healthy c ontrols (group A) and 21 patients with nonthyroidal illness (NTI; grou p B). Ten patients had normal plasma T-3 and T-4 values (group B-1), a nd 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 t hat 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 subgrou ps Bl and By. TSH pulse frequency was not different between NTI patien ts and controls (similar to 10 pulses/24 h). The mean 24-h TSH concent ration was significantly related to the mean 24-h TSH pulse amplitude but not to the mean 24-h TSH pulse frequency. The absolute nocturnal T SH 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 pati ents with decreased plasma T-3 (group B-2) was associated with a loss of the usual nocturnal increase nocturnal increase in TSH pulse freque ncy 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 c ompletely absent. The changes in nocturnal TSH secretion in NTI resemb le those found in central hypothyroidism, suggesting that features of central hypothyroidism are involved in the euthyroid sick syndrome.