Nutrition influences thyroid function at the level of TSH secretion, at the level of monodeiodination, and possibly elsewhere. In order to study the effect of starvation on TSH secretion, 8 healthy male volunteers fasted for 30 h and were then refed with 800 kcal. Refeeding was performed at 19.00 h and blood was sampled at 20 min intervals until midnight. Control experiments were performed in the same subjects both when they were normally fed and when the starvation period was prolonged a further 5 h until midnight. Starvation decreased serum TSH levels to below 1 mU/1, and without refeeding the nocturnal peak of the TSH nycthemeral rhythm was abolished. With refeeding serum TSH tended to increase towards midnight and was significantly higher than during starvation. However, the serum TSH levels remained significantly below those at the same time of the day in the absence of a preceding starvation period. Serum T3 levels were significantly lower than in the fed state. The mean values were 1.84 +/- 0.03 vs 2.30 +/- 0.06 nmol/l (120 +/- 2 vs 150 +/- 4 ng/100 ml, mean +/- SEM P less than 0.01). Refeeding did not result in a measurable change in serum T3 concentration (1.80 +/- 0.05 nmol/l; 120 +/- 3 ng/100 ml, mean +/- SEM, n.s.). The contrary was true for rT3 levels which increased in starvation and tended to fall with refeeding, but this decrease was not significant. As glucocorticoids have been implicated in the control of monodeiodination and TSH secretion, serum cortisol levels were also measured. They did not differ during the 3 experimental periods.(ABSTRACT TRUNCATED AT 250 WORDS)