Information regarding cationic amino acid transport systems in thyroid is limited to Northern blot detection of y⁺LAT1 mRNA in mouse. The aim of this study was to investigate cationic amino acid transport in PC Cl3 cells, a thyroid follicular cell line derived from normal Fisher rat retaining many features of normal differentiated follicular thyroid cells. We provide evidence that in PC Cl3 cells plasmalemmal transport of cationic amino acids is Na⁺-independent and occurs, besides diffusion, by contribution of high-affinity, carrier-mediated processes. Carrier-mediated transport is via y⁺, y⁺L and b^0,+ systems, as assessed by L-arginine uptake/kinetics, inhibition of L-arginine transport by N-ethylmaleimide and neutral amino acids and L-cystine transport studies. y⁺L and y⁺ systems account for the highest transport rate (with y⁺L > y⁺), while b^0,+ for a residual fraction of the transport. Uptake data correlate to expression of the genes encoding for CAT-1, CAT-2B, 4F2hc, y⁺LAT1, y⁺LAT2, rBAT and b^0,+AT, an expression profile that also emerges from the rat thyroid gland. In PC Cl3 cells cationic amino acid uptake is under TSH/cAMP control (with transport increasing by increasing TSH concentration) and up-regulation of CAT-1, CAT-2B, 4F2hc/y⁺LAT1 and rBAT/b^0,+AT occurs at the mRNA level under TSH stimulation. Our results provide the first description of an expression pattern of cationic amino acid transport systems in thyroid cells. Furthermore, we provide evidence that extracellular L-arginine is a crucial requirement for normal PC Cl3 cell growth and that long term L-arginine deprivation negatively influences CAT-2B expression, as it correlates to reduction of CAT-2B mRNA levels.