ABSTRACT
Thyroid hormone plays an important role in the regulation of the energy expenditure of the organism. This and many other actions of thyroid hormone are initiated by interaction with receptor proteins located in the cell nucleus of different tissues which control the transcription of thyroid hormone-responsive genes. These receptors have recently been identified as the products of the c-erbA protooncogenes. 1 It is generally accepted that 3,3′ ,5-triiodothyronine (T3) is the predominant bioactive form of thyroid hormone. Thyroxine (3,3′ ,5,5′-tetraiodothyronine, T4) has little intrinsic metabolic activity and serves mainly as a prohormone for T3. The thyromimetic activity of all other naturally occurring iodothyronines, including 3,3′ ,5′-triiodothyronine (reverse T3, rT3), is negligible. The structure-bioactivity relationship for iodothyronines corresponds closely with the binding affinity of these compounds for the nuclear receptor. 1
