High iodine levels inhibit iodide oxidation and organification. Additionally, iodine excess inhibits thyroglobulin proteolysis (this is the principal mechanism for the antithyroid effect of inorganic iodine in patients with thyrotoxicosis).
Although T3 is more biologically active than T4, the major production of T3 actually occurs outside of the thyroid gland. The majority of T3 is produced by peripheral conversion from T4 in a deiodination reaction involving a specific enzyme which removes one iodine from the outer ring of T4.
In order to attain normal levels of thyroid hormone synthesis, an adequate supply of iodine is essential. The recommended minimum intake of iodine is 150 micrograms a day. Intake of less than 50 micrograms a day is associated with goiter.
Apr 18, 2014. Thyroid hormones are critical determinants of brain and somatic development in infants and of metabolic activity in adults; they also affect the.
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. THYROID HORMONE BIOSYNTHESIS There are two biologically active thyroid hormones: thyroxine (T4) and 3,5,3'-triiodothyronine (T3) ( figure 1 ).
If a di-iodotyrosine and a mono-iodotyrosine are coupled together, the result is the formation of tri-iodothyronine (T3). From the perspective of the formation of thyroid hormone, the major coupling reaction is the di-iodotyrosine coupling to produce T4.
The T3 and T4 released from the thyroid by proteolysis reach the bloodstream where they are bound to thyroid hormone binding proteins. The major thyroid hormone binding protein is thyroxin binding globulin (TBG) which accounts for about 75 of the bound hormone.
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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.
This reaction results in either a mono-iodinated tyrosine (MIT) or di-iodinated tyrosine (DIT) being incorporated into thyroglobulin. This newly formed iodothyroglobulin forms one of the most important constituents of the colloid material, present in the follicle of the thyroid unit.
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).
Either too little or too much thyroid hormones will bring disorders to the body. Synthesis. Iodine is the raw material of the thyroid biosynthesis. Once taken up.
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".) ANATOMY The thyroid gland weighs 10 to 20 grams in normal adults in the United States 1.
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.