Calcitonin inhibits TRH-induced TSH secretion

The authors investigated in 14 healthy volunteers the effect of a single dose of 50 MRCU of synthetic salmon calcitonin on plasma levels of thyroxine (T4), triiodothyronine (T3), thyrotropin (TSH), calcium, phosphorus and magnesium and on the thyrotropic secretory reserve assessed by means of the TRH test. T4 and T3 levels did not change significantly within 120 min after administration of calcitonin. TSH levels during the 60th minute declined insignificantly. TRH stimulated TSH levels after administration of calcitonin were significantly lower, as compared with control values (p less than 0.01). The thyrotrophic secretory reserve determined as the difference of TSH values at rest and after stimulation was significantly inhibited by calcitonin (p less than 0.01). The phosphorus and magnesium levels did not change significantly after calcitonin, the calcium levels displayed only a slightly declining trend. No correlation was revealed between the secretory response of TSH and calcium, phosphorus and magnesium levels. The inhibitory action of calcitonin on TSH secretion is probably not mediated by changes in the extracellular concentration of these ions but is most probably the consequence of the direct influence on the secretory mechanism inside the thyrotrophic cell.