The treatment by lithium is known to involve certain endocrine complications. Those concerning the thyroid function, with risk of a frank hypothyroidy, are the most important. Aiming to appreciate the frequence and the intensivity of the endocrine effects of lithium, the thyroid parameters and the steady state of the hypothalamo-pituitary-thyroid axis were tested using the TRH test in 52 patients with maniaco-depressive psychosis with special attention to TSH, prolactin and growth hormone: 24 out of them were treated for 1 month to 6 years by lithium; the 28 others were considered as controls. The lithium treatment involves a decrease in the free thyroxine index (1.78 +/- 0.09 vs 2.16 +/- 0.09; p less than 0.01), an increase in the mean baseline TSH level (5.80 +/- 1.49 vs 2.70 +/- 0.24 microU/ml; p less than 0.05) and a noteworthy increase in the TSH responsiveness to TRH (22.7 +/- 2.14 vs 9.75 +/- 1.63 microU/ml; p less than 0.005). The TSH supranormal responses were neither correlated with the length of the treatment nor with the age of the patients. They appear as the consequence of a decrease in the thyroidal hormone secretion. The basal and stimulated prolactinemias remain comparable in the two groups of patients and no response of growth hormone occured after TRH. The TRH test must be considered as a useful complement for the surveillance of the patients treated with lithium because it permits to diagnose early the lithio-induced thyroid dysfunction.