Human SLC5A5 encodes the Na+/I- symporter NIS which is localised in the basolateral membrane of thyrocytes facing the bloodstream where it mediates iodide accumulation into these cells. Defects in SLC5A5 can cause hyroid dyshormonogenesis 1 (TDH1; MIM:274400), a disorder characterised by the inability of the thyroid to maintain a concentration difference of readily exchangeable iodine between the plasma and the thyroid gland (termed iodine trapping) leading to congenital hypothyroidism. Mutations in SLC5A5 that can cause TDH1 include T354P, V59E, G395R, C272*, G93R and R124H (Fujiwara et al. 1997, Pohlenz et al. 1997, Kosugi et al. 1998, 1999).