Specific triiodothyronine binding sites in the anterior pituitary of the rat

Studies with L-[¹²⁵I]triiodothyronine and L-[ ¹²⁵I]thyroxine, and with equilibrium dialysis of plasma proteins indicate that rat pituitary binds L-triiodothyronine 9.8 times as strongly as it does L-thyroxine. Injection of even small doses of nonradioactive L-triiodothyronine reduces the pituitary/plasma ratio of radioactive L-triiodothyronine, an indication of the existence of pituitary binding sites with a limited capacity for L-triiodothyronine. Limited capacity binding sites for L-thyroxine could not be demonstrated.