Fig. 3.

Models for RTHβ and RTHα. a RTHβ occurs in tissues expressing TRβ and causes a rise in serum T₃ and T₄ levels due to impaired negative feedback of the hypothalamic/pituitary/thyroid axis. b RTHα occurs in tissues expressing predominantly TRα (bone, gut, and heart) and causes symptoms shown in Table 1. In contrast to RTHβ, there is no defect in the negative feedback of the HPT axis by TH. However, patients have increased serum T₄/ T₃ ratios suggesting that a downstream effect such as increased deiodinase 1 (Dio 1) activity may occur