Syndromes of Reduced Sensitivity to Thyroid Hormone

This chapter discusses syndromes related to the reduced sensitivity to thyroid hormone which include a group of disorders with abnormalities in the level of serum thyroid stimulating hormone (TSH) and other thyroid hormone (TH) levels. Resistance to thyroid hormone (RTH) is stated to be a syndrome of reduced end-organ responsiveness to TH and is associated with mutations in TH receptors (TRs). An overview of described and putative defects of the syndromes is given and includes thyroid hormone cell transport defect, thyroid hormone metabolism defect and abnormal hormone transfer to the nucleus. The genetic pathophysiology of the syndromes is further discussed and in the case of RTH, mutations in TRβ gene located on chromosome 3 causes the syndrome. In thyroid hormone metabolism defect syndrome, a defect in the gene that codes for selenocysteine insertion sequence-binding protein 2 (SBP2) is involved and is manifested as a global deiodination defect giving rise to low triiodothyronine (T3) and high tetraiodothyronine thyroxine (T4) and reverse T3 (rT3). The treatment options for these syndromes are also mentioned.