21.11
Synthesis and Regulation of Thyroid Hormones
Low blood levels of the thyroid hormones — triiodothyronine (T3) and thyroxine (T4) — signal the hypothalamus to release the thyrotropin-releasing hor…
Low blood levels of the thyroid hormones — triiodothyronine or T3 and thyroxine or T4 —signal the hypothalamus to release the thyrotropin-releasing hormone or TRH.
TRH then reaches the pituitary gland and stimulates the release of thyroid-stimulating hormone or TSH into the bloodstream.
Upon reaching the thyroid gland, TSH stimulates the active uptake of iodide ions from the blood by the follicular cells.
The ions diffuse to the apical surface of the cells and are oxidized to iodine. The iodine is then added to tyrosine residues of the thyroglobulin protein stored in the lumen colloid, forming monoiodotyrosine or T1 and diiodotyrosine or T2.
The coupling of one molecule of T1, with T2 forms a T3 molecule, whereas two T2 molecules combine together to form T4.
Finally, endocytosis and enzymatic cleavage releases the T3 and T4 in the follicular cells. These hormones then cross into the blood vessel for transport to target tissues.
An increase in blood levels of T3 and T4 temporarily inhibits the production of TRH and TSH, regulating the level of thyroid hormones in the body.
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Q1: What triggers the release of thyroid hormones when blood levels drop?
Low blood levels of thyroid hormones T3 and T4 signal the hypothalamus to release thyrotropin-releasing hormone (TRH). TRH travels to the pituitary gland, stimulating the release of thyroid-stimulating hormone (TSH) into the bloodstream. TSH then reaches the thyroid gland to initiate hormone synthesis and release.
Q2: How does TSH stimulate thyroid follicular cells to produce hormones?
TSH stimulates active uptake of iodide ions from the blood by follicular cells. These ions diffuse to the apical surface and are oxidized to iodine. The iodine is then incorporated into tyrosine residues of thyroglobulin protein stored in the colloid, initiating the synthesis of thyroid hormones.
Q3: What is the role of iodine in forming T3 and T4 molecules?
Iodine is added to tyrosine residues of thyroglobulin protein, forming monoiodotyrosine (T1) and diiodotyrosine (T2). One T1 molecule couples with one T2 molecule to form T3, while two T2 molecules combine to form T4. These coupling reactions are essential for creating functional thyroid hormones.
Q4: How are T3 and T4 hormones released from follicular cells into the bloodstream?
Endocytosis brings thyroglobulin protein containing T3 and T4 into follicular cells from the colloid. Enzymatic cleavage then releases the mature T3 and T4 hormones from the protein. These hormones cross into blood vessels for transport to target tissues throughout the body.
Q5: How do elevated thyroid hormone levels regulate their own production?
Increased blood levels of T3 and T4 temporarily inhibit the production of TRH and TSH through negative feedback. This reduces stimulation of the thyroid gland, decreasing hormone synthesis and release. This feedback mechanism maintains stable thyroid hormone levels in the bloodstream.
Q6: What is the relationship between the hypothalamus, pituitary gland, and thyroid gland?
The hypothalamus, pituitary gland, and thyroid gland form an integrated control system. The hypothalamus releases TRH, which stimulates the pituitary to release TSH, which then stimulates the thyroid to produce T3 and T4. This hierarchical pathway demonstrates chemical signaling in the endocrine system.
Q7: Why is iodide uptake by thyroid follicular cells considered an active process?
Active uptake of iodide ions requires energy expenditure by follicular cells to move ions against their concentration gradient from the blood. This active transport mechanism ensures sufficient iodide accumulation for thyroid hormone synthesis, even when blood iodide levels are low or depleted.