ABSTRACT
Biosynthesis of Thyroid Hormones In response to physiologic need, hypothalamic TSH-releasing hormone (also called thyrotopin-releasing hormone, TRH) stimulates release of TSH from thyrotropes of the anterior pituitary (Part B). TSH is required for the uptake of iodide (I-) used for hormone biosynthesis within thyroid follicular cells. Iodine (I) is converted to I-
before being absorbed from the small bowel, then I-circulates in plasma, bound to plasma proteins (though some is free). Thyroid cell membranes contain a Na+/I-symporter (NIS) that transports Na+
and I-into cells, producing intracellular I-concentrations 20-40 x > than plasma. Although the NIS is also found in salivary glands, the gastric mucosa, placenta, ciliary body, choroid plexus, and mammary glands, these latter tissues are not TSH-sensitive, and they do not organify or store I-. Thyroid tissue ends up with > 90% of the body I-
pool. Monovalent anions such as perchlorate (ClO4-), pertechnetate (TcO4-) , and thiocyanate SCN-compete with I-for uptake, they are also concentrated by thyroid follicular cells, and technetiumlabeled (99mTc) pertechnetate is used clinically in thyroid scans. KTcO4 and KSCN are also used to treat I--induced hyperthyroidism.
