TSH secretion, with particular regard to the nocturnal TSH surge, was evaluated in 115 subjects with non-toxic nodular goiter. All patients were clinically and biochemically euthyroid. After 18-36 months of follow-up (mean, 24 months), hyperthyroidism occurred in 21 (18%; group 1), while the remaining 94 remained euthyroid (82%; group II). The analysis of hormonal data at the time of first observation showed that the 2 groups had similar total and free T4 and T3 serum concentrations. Morning serum TSH values in group I were lower than those in group II patients (0.6 +/- 0.1 vs. 1.1 +/- 0.1 mU/L; P less than 0.001); this difference was even more striking for the nocturnal values (0.6 +/- 0.1 vs. 2.2 +/- 0.2 mU/L; P less than 0.0001); nocturnal values were significantly lower than morning values in group II, but not in group I. The mean peak TSH value after TRH was also significantly reduced in group I (5.5 +/- 0.4 vs. 9.2 +/- 0.7 mU/L; P less than 0.001). Morning TSH values in group II did not differ from those in controls (1.3 +/- 0.1 mU/L), whereas nocturnal and TRH stimulated peak TSH values were slightly but significantly lower. The nocturnal serum TSH values in control subjects were 62-390% higher than morning values. The nocturnal TSH surge was abolished in 18 of 21 (86%) group I patients and in 7 of 94 (8%) group II patients. TRH testing resulted in an absent or blunted TSH responses in 5 subjects in group I and 6 in group II. Analysis by the Galen and Gambino predictive model; comparing the abolition of the nocturnal TSH surge and the abnormal TRH test as predictors of the subsequent occurrence of hyperthyroidism, showed that the former had higher sensitivity (86% vs. 24%) and predictivity (72% vs. 45%). In conclusion, the results of the present study demonstrate that the evaluation of the nocturnal TSH surge may be useful in identifying patients with nontoxic nodular goiter in whom hyperthyroidism may eventually occur. Patients who lack the nocturnal serum TSH surge are more prone to develop thyroid hyperfunction; their thyroid status should, therefore, be more carefully and frequently monitored.