Glucose transport by FRTL-5 cells, a rat thyroid cell line, was found to be TSH dependent. The effect of TSH on the uptake of 2-deoxy-D-glucose, a nonmetabolizable glucose analogue, was prompt, being 200% over basal value after 10 min and maximal after 12 h (600-700% increase). The TSH effect was dose dependent, with half-maximum stimulation at 10 microU TSH/ml, and maximum stimulation at 1 mU TSH/ml. TSH enhanced also the uptake of 3-O-methyl-D-glucose by FRTL-5 cells. The TSH activation of glucose transport had the following characteristics: it was mimicked by (Bu)2-cAMP (1 mM) and by agents that increase cAMP levels in thyroid cells, such as forskolin (10 microM) and cholera toxin (50 micrograms/ml); it involved the facilitated glucose transport system in that it was inhibited in a dose-related manner by both cytochalasin B and phloretin; it showed a glucose stereochemical sensitivity, being affected by D-glucose and 3-O-methyl-glucose, and not by L-glucose; it was characterized by an increase in the maximum velocity (Vmax) of glucose uptake (from 15.3 to 66.0 fmol/min X micrograms DNA) without change in the Michaelis-Menten constant (Km) (5.3 mM); the effect on the Vmax was due to an increase in the number of surface glucose transporters as indicated by the enhancement of the D-glucose-sensitive fraction of [3H]cytochalasin B binding sites that in thyroid plasma membranes of cells exposed to TSH for 2 and 8 h, increased from 5.0 (basal value) to 10.4 and 23.1 pmol/mg protein, respectively. These data indicate that in FRTL-5 cells TSH stimulates the glucose transport system by an enhancement of the number of functional glucose transporters in the thyroid plasma membrane.