The human hormone, thyrostimulin is within the glycoprotein family and consists of the two sub-units; alpha- and beta-. These subunits are bonded noncovalently into a heterodimer. The common alpha-subunits combine with four specific beta-subunits. This leads to four biologically human hormones; FSH, LH, TSH and CG.
The hormones FSH, LH and TSH express in the anterior pituitary and have been shown to be essential for the coordinated endocrine regulation in the hypothalamus- pituitary axis. The TSH receptor has shown to activate specific G protein-coupled receptors in the thyroid, and LH and TSH in the gonads. The family of heterodimeric glycoproteins have only been found in vertebrates. They are conserved in organisms such as primitive rayfin fish to humans, in both primary sequences and functional characteristics.
Thyrostimulin Mechanism & Function
Corticotroph-derived glycoprotein hormone (CGH), also classes as a thyrostimulin due to its thyroid-stimulating activity. CGH is a non-covalent heterodimer consisting of a glycoprotein hormone alpha 2 (GPHA2) and glycoprotein hormone beta 5 (GPHB5). The recombination of the GPHA2 and GPHB5 hormones activate human TSH receptors, however it does not activate LH and FSH receptors. From radioligand assays, CGH has shown to have a high affinity to TSH receptors. It has also shown to stimulate cAMP production and thymidine incorporation in thyroid cells. From in-vivo studies, increases in serum thyroxine levels have been observed in TSH-suppressed rats. Due to the expression of thyrostimulin in the anterior pituitary which is known to express TSH receptors leads to the assumption that the mechanism is paracrine.