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Thyroid hormone synthesis and physiology

Douglas S Ross, MD
Section Editor
David S Cooper, MD
Deputy Editor
Jean E Mulder, MD


Thyroid hormones are critical determinants of brain and somatic development in infants and of metabolic activity in adults; they also affect the function of virtually every organ system. Thyroid hormones must be constantly available to perform these functions. To maintain their availability there are large stores of thyroid hormone in the circulation and in the thyroid gland. Furthermore, thyroid hormone biosynthesis and secretion are maintained within narrow limits by a regulatory mechanism that is very sensitive to small changes in circulating hormone concentrations.

The processes of thyroid hormone synthesis, transport, and metabolism, and the regulation of thyroid secretion will be reviewed here. The actions of thyroid hormone are discussed elsewhere. (See "Thyroid hormone action".)


The thyroid gland weighs 10 to 20 grams in normal adults in the United States [1]. Thyroid volume measured by ultrasonography is slightly greater in men than women, increases with age and body weight, and decreases with increasing iodine intake [2].

Microscopically, the thyroid is composed of spherical follicles, each composed of a single layer of follicular cells surrounding a lumen filled with colloid (mostly thyroglobulin). When stimulated, the follicular cells become columnar and the lumen is depleted of colloid; when suppressed, the follicular cells become flat and colloid accumulates in the lumen.


There are two biologically active thyroid hormones: thyroxine (T4) and 3,5,3'-triiodothyronine (T3) (figure 1). They are composed of a phenyl ring attached via an ether linkage to a tyrosine molecule. Both have two iodine atoms on their tyrosine (inner) ring. They differ in that T4 has two iodine atoms on its phenyl (outer) ring, whereas T3 has only one. The compound formed if an iodine atom is removed from the inner ring of T4 is 3,3',5'-triiodothyronine (reverse T3 [rT3]), which has no biological activity.

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Literature review current through: Nov 2017. | This topic last updated: May 31, 2016.
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