Rapid stimulation of type I 5′-deiodinase in rat pituitaries by 3,3′,5-triiodo-l-thyronine

Abstract The negative feedback control of thyrotropin production by the anterior pituitary involves local 5′-deiodination of l -thyroxine (L-T 4 ) to the active thyroid hormone 3,3′,5-triiodo- l -thyronine (T 3 ) by two 5′-deiodinase isozymes which are distinctly regulated by thyroid hormone. T 3 rapidly increased steady-state mRNA levels and activity of type I iodothyronine-5′-deiodinase (5′DI) in rat anterior pituitary and in reaggregate cultures of anterior pituitaries. Type II 5′-deiodinase activity determined in parallel with 3,3′,5′-triiodo- l -thyronine (rT 3 ) as substrate was markedly lower than that of 5′DI. 5′DI mRNA levels and activity were higher in anterior pituitaries of female compared to male rats. Neither gender differences nor T 3 stimulation of 5′DI activity were found in the posterior part. These data demonstrate a T 3 dependent expression of 5′DI in euthyroid anterior pituitary and suggest that this isozyme serves a major function within the complex network of thyroid hormone homeostasis.