Iodide uptake, thyroid hormone synthesis

The thyroid gland also regulates its uptake of iodide and thyroid hormone synthesis by intrathyroidal mechanisms that are independent of TSH. These mechanisms are primarily related to the level of iodine in the blood. Large doses of iodine inhibit iodide organification (Wolff-Chaikoff block, see Figure 38-1). In certain disease states (eg, Hashimoto s thyroiditis), this can inhibit thyroid hormone synthesis and result in hypothyroidism. Hyperthyroidism can result from the loss of the Wolff-Chaikoff block in susceptible individuals (eg, multinodular goiter). 

FIGURE 10.21 Iodine uptake and thyroid hormone synthesis by the thyroid gland. Thy-roglobulin is broken down to yield T4 and T3 as well as iodotyrosine byproducts. These byproducts are further broken down in the thyroid to yield iodide. 

Figure 56.1 This figure gives a schematic representation of the foiiicuiar thyroid ceii. Thyroid hormone synthesis takes piace across a number of different steps, starting from iodide uptake and ending with the reiease of free thyroid hormones in the bioodstream.

Iodide, ingested from food, water, or medication, is rapidly absorbed and enters an extracellular fluid pool. The thyroid gland removes about 75 meg a day from this pool for hormone synthesis, and the balance is excreted in the urine. If iodide intake is increased, the fractional iodine uptake by the thyroid is diminished. 

Correct answer = O. Propylthiouracil blocks the synthesis of the thyroid hormones, but does not affect the uptake of iodide, proteolytic cleavage of thyroglobulin, or the release of hormones from the thyroid gland. The thyroid hormones inhibit the secretion of thyroid-stimulating hormone from the anterior pituitary. ... 

In thyroid cells in culture, ceilcitriol reduces production of cAMP in response to thyr oid stimulating hormone by a nuclear- action on the synthesis of G-protein subunits. However, it also reduces the responsiveness to cAMP, emd attenuates cell growth and iodide uptake in response to thyroid stimulating hormone, with a rapid time course from direct action on protein kinase A (Berg andHaug, 1999). 

Thyroid stimulating hormone (TSH) thyro-trophin, a glycoprotein of the anterior pituitary, controls the synthesis and release of thyroid hormone from the gland, and also the uptake of iodide. There is a negative feedback of thyroid hormones on both the hypothalamic secretion of TRH and pituitary secretion of TSH. 

The first measurable effect of TSH on thyroid hormone metabolism is increased secretion, which is detectable within minutes. AU phases of hormone synthesis and secretion are eventually stimulated iodide uptake and organification, endocytosis, and proteolysis of thyroglobulin. There also is increased vascularity of the gland and hypertrophy and hyperplasia of thyroid cells. 

The mechanism leading to more hypothyroidism with higher iodine intake has not been fully elucidated. A high iodine load to the thyroid leads to inhibition of many thyroidal processes involved in hormone synthesis and secretion. Normally the thyroid will escape from this inhibition via downregulation of NIS and a subsequent fall in thyroidal iodide uptake (Eng et al., 1999). Apparently, this escape process is not fuUy effective in many people, especially people with a partly defective function of the thyroid because of thyroid autoimmunity or previous thyroid disease. 

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