SLC5A5 and congenital hypothyroidism: In line with this, the thyroid-stimulating hormone (TSH) is the primary hormonal regulator of thyroid NIS expression [29], and pathogenic variants in the TSH receptor-coding TSHR gene have been identified in patients with congenital hypothyroidism showing hypoplastic or normal-sized eutopic thyroid glands with reduced to absent radionuclide uptake on thyroid scintigraphy [30,31].