Structure and Regulated Expression of the TSH Receptor Gene: Differences and Similarities to Gonadotropin Receptors

Glycoprotein hormones—lutropin/chorionic gonadotropin (LH/CG), follicle stimulating hormone (FSH), and thyrotropin (TSH)—have a high degree of primary and tertiary structure similarity, yet are target tissue specific (1–5). The structural similarities of the hormones are complemented by functional similarities (1–5). Thus, all are well recognized to activate the cAMP signal transducing system, and all modulate both growth and differentiation of their respective target tissues. It is increasingly evident, in addition, that all depend on the action of other hormones, such as insulin and insulin-like growth factor I (IGF-I), to regulate growth and differentiated function (6–8).