Inhibition by somatostatin of basal levels of serum thyrotropin (TSH) in normal men.

To elucidate the effect of somatostatin on basal, unstimulated TSH secretion 6 young healthy nonobese men were studied with blood samples every 15 min from 2300 h to 0500 h. Somatostatin 1 mg was indused over a 2-h period from 0100 h. One of the subjects was also studied with half hourly blood samples throughout a 26-h period without receiving somatostatin. In all cases the high night levels of serum thyrotropin (TSH) were significantly suppressed by somatostatin. The fall of TSH continued throughout the two hour infusion period. Immediately upon termination of the infusion serum TSH increased rapidly to preinfusion values. The 26-h control study showed the typical diurnal pattern of serum TSH with low values in the daytime and high values at night. The serum triiodothyronine (T3) and thyroxine (T4) levels were not changed by somatostatin infusion. From these results we suggest that endogenous somatostatin may be of physiological importance in the regulation of the TSH secretion.