The activity of the thyroid hormones, L-thyroxine (T4; ) and 3,3',5-triiodo-L-thyronine (T3; ), is mediated by thyroid hormone nuclear receptors TRα and TRβ. Gene knockout studies in mice indicate that TRα modulates the effect of thyroid hormone on calorigenesis and on the cardiovascular system, while TRβ plays a role in the development of the auditory system and in the negative feedback of thyroid stimulating hormone by T3 in the pituitary. Thyroid hormone receptor antagonist (1-850) competitively blocks T3 binding to both TRα and TRβ (IC50s = 1.5 μM), demonstrating ~1,000-fold lower affinity for TRα in intact cells compared with T3. This compound is specific, showing no effect on the activity of the related retinoic acid receptor α.
