Effects of TSH on iodide transport by mouse thyroid lobes in vitro

Thyroidal iodide concentration by in vitro mouse thyroid lobes was studied by determining the equilibrium tissue/medium iodide, concentration ratio (T/M[I-]), THYROIDAL I- influx, and thyroidal I-efflux in the presence and absence of ClO4-. Chronic thyroid stimulation by low dietary iodine increased the T/M[I-], and the increase was linearly related to I- influx. There was no difference in efflux rate when animals fed low and high iodine diets were compared, although the efflux with C104- added to block influx was increased by low iodine diet. Addition of TSH in vitro caused a delayed fall in T/M[I-] with a similar TSH concentration-dependence as colloid droplet formation. The TSH effect was mimicked by exogenous cyclic AMP but could be dissociated from stimulation of hormone release by colchicine. Thyroidal I- efflux was increased up to 20-fold by C104-. In the presence of C104- short-term and equilibrium-labeled I- exited at the same rate, but in the absence of C104- short-term-labeled I- efflux was consistently higher. The TSH-induced fall in T/M I- could be accounted for by increased iodide efflux. As TSH also increased efflux when influx was blocked by C104-, the TSH effect would seem due to an increased intrinsic follicular leakiness.