Direct effects of protirelin (TRH) on cultured porcine thyrocytes.

In several cases of thyroid adenoma in which no TSH was detectable in the serum, a paradoxical decrease of PB123I and its conversion rate Q (PB123I/serum total radioactivity) was observed after infusion of protirelin (TRH). The possibile direct effect of TRH on thyrocytes was therefore studied in vitro. Porcine thyroid cells were cultured in NCTC-135 medium supplemented with a serum substitute and insulin. TRH and some of its analogs enhanced the incorporation of [3H]thymidine into DNA and the uptake of radioiodide into thyrocytes. In the presence of TSH, TRH did not stimulate radioiodine uptake, whereas incorporation of [3H]thymidine into DNA was not antagonized by TSH. Thus, in this culture system, TRH had a direct effect on both growth and metabolism of thyrocytes. TSH can modulate these effects.