ABSTRACT
The main regulation of the thyroid gland involves a positive control by pituitary thyrotropin. Thus, in classical physiology, the pituitary thyrotrophs synthesize and secrete Thyroid-stimulating hormone (TSH) and are under tonic stimulation by the hypothalamic thyrotropin-releasing hormone (TRH) and negative feedback by thyroid hormone. Reduced serum TSH levels is followed by diminished thyroid function and reduction in the weight and cell mass of the gland. The TSH receptor is a dimer consisting of a glycosylated, 55-kilodalton (kDa) a subunit connected by disulfide bond(s) to a 34-kDa B subunit. According to this conception the A subunit is responsible for the interaction with TSH, whereas subunit B anchors the receptor in the basal plasma membrane. The primary structure of the TSH receptor, deduced from the sequence of its cDNA, confirmed that it belongs to the G-coupled receptor family. It contains seven segments with a length compatible with that of transmembrane segments.
