Thyroxine and Triiodothyronine Are Synthesized and Secreted by the Thyroid Follicle

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The lobes of the thyroid gland consist of aggregates of many spherical follicles, lined by a single layer of epithelial cells (Fig. 33.1). The apical membranes of the follicular

The Thyroid Spherical Follicles
A cross-sectional view through a portion of the human thyroid gland.

cells, which face the lumen, are covered with microvilli. Pseudopods formed from the apical membrane extend into the lumen. The lateral membranes of the follicular cells are connected by tight junctions, which provide a seal for the contents of the lumen. The basal membranes of the follicular cells are close to the rich capillary network that penetrates the stroma between the follicles.

The lumen of the follicle contains a thick, gel-like substance called colloid (see Fig. 33.1). The colloid is a solution composed primarily of thyroglobulin, a large protein that is a storage form of the thyroid hormones. The high viscosity of the colloid is due to the high concentration (10 to 25%) of thyroglobulin.

The thyroid follicle produces and secretes two thyroid hormones, thyroxine (T4) and triiodothyronine (T3). Their molecular structures are shown in Figure 33.2. Thy-roxine and triiodothyronine are iodinated derivatives of the amino acid tyrosine. They are formed by the coupling of the phenyl rings of two iodinated tyrosine molecules in an ether linkage. The resulting structure is called an iodothyronine. The mechanism of this process is discussed in detail later.

Thyroxine contains four iodine atoms on the 3, 5, 3', and 5' positions of the thyronine ring structure, whereas triiodothyronine has only three iodine atoms, at ring positions 3, 5, and 3' (see Fig. 33.2). Consequently, thyroxine is usually abbreviated as T4 and triiodothyronine as T3. Because T4 and T3 contain the element iodine, their synthesis by the thyroid follicle depends on an adequate supply of iodine in the diet.

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