TH deficiency and excess are associ

TH deficiency and excess are associated with typical symptoms and signs reflecting the effects of global lack and excess of the hormone, respectively, on all body tissues. A deviation from this rule was recognized with the identification of the RTH syndrome. Usually caused by heterozygous mutations in the TRβ gene, subjects have high levels of TH without TSH suppression. This paradox extends to clinical and biochemical observations, suggesting that TH deficiency, sufficiency, and excess depend on the level of TRβ gene expression in various tissues.5 The syndrome of TH cell transport defect exhibits a similar paradox, as subjects have a high serum T3 concentration but the uptake of TH is not uniform in all tissues and cell types.14