Lack of stimulating effect on thyrotropic and lactotropic secretion during prolonged calcitriol administration.

The stimulating effect of calcitriol, administered in the dose of 3 micrograms/day for four days, on the thyrotropic and lactotropic secretion has been proved in earlier studies in healthy humans. The present study was undertaken to confirm or eliminate the stimulatory action of the drug using increasing dosages of 0.75-1.50 micrograms/day for one month. In spite of the fact, that the biologically efficient levels of calcitriol were attained (confirmed indirectly by the rise of urinary calcium excretion, p less than 0.01), the stimulating effect of calcitriol on the basal and TRH induced thyrotropic and lactotropic secretion has not been manifested. In conclusion, the clinical importance of stimulating effect of calcitriol on the thyrotropic and lactotropic secretion is neglected, if only commonly accepted therapeutic dosages up to 1.50 micrograms/day are used.