. MCT1 and its accessory protein CD147 are differentially regulated by TSH in rat thyroid cells. Am J Physiol Endocrinol Metab 285: E1223-E1229, 2003 10.1152/ ajpendo.00172.2003In thyroid cells, basal and TSH-stimulated glycolysis is associated with lactic acid efflux. In this report, we address whether monocarboxylate transporters (MCTs) are present in thyroid tissue for exporting excess lactic acid generated by aerobic glycolysis. Using immunostaining techniques, we show that MCT4 localizes with its accessory protein CD147 in the basolateral membrane of rat thyroid follicular cells. In cultured rat thyroid (FRTL-5) cells, MCT1 rather than MCT4 is expressed. CD147 colocalizes and coimmunoprecipitates with MCT1. TSH upregulates MCT1/ CD147 expression as a function of time through a cAMPdependent mechanism as forskolin reproduces the effect of TSH. TSH enhances protein expression of both MCT1 and CD147 in FRTL-5 cells. Whereas MCT1 protein expression is controlled at the level of transcription, CD147 protein expression is regulated by a posttranscriptional mechanism. Results of these studies suggest that hormone stimulation of lactate transport is mediated by regulating MCT1 transcription. monocarboxylate transporter 1; monocarboxylate transporter 4; thyroid gland; thyroid-stimulating hormone; FRTL-5 cells; glycolysis; lactate IN THE THYROID GLAND, the primary metabolic fuels are glucose and free fatty acids (1,7,8). Oxidative phosphorylation uses mainly free fatty acids as substrate and accounts for Ͼ80% of ATP production. Mitochondrial respiration is primarily regulated by the ADP supply, and the stimulatory effect of TSH and cAMP is secondary to enhanced cellular processes that consume ATP. Aerobic glycolysis accounts for 70% of the glucose metabolized by the thyroid cell and supplies almost 20% of the cellular ATP (reviewed in Ref.