Calcitonin from parafollicular c cells triiodothyronine, thyroxine str

Calcitonin (from parafollicular c cells) triiodothyronine, thyroxine structure

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thyroid hormones T3 T4 chemical structure calcitonin parafollicular C cells authoritative

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Source: Parafollicular cells, also called C cells, of the thyroid gland.
Structure: A single-chain polypeptide of 32 amino acids.
- It has a disulfide bridge between amino acids 1 and 7.
Secretion stimulus: Mainly increased ionized serum Ca²⁺. Gastrin can also stimulate release.
Main actions: Lowers serum calcium and phosphate by:
- inhibiting osteoclast-mediated bone resorption
- reducing renal reabsorption of calcium and phosphate
It is not the principal regulator of calcium homeostasis in adult humans. PTH and vitamin D are more important.
Clinical point: calcitonin is a useful marker for medullary thyroid carcinoma, a tumor of C cells.

Katzung's Basic and Clinical Pharmacology, 16th Edition, p. 395; Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 250.

Hormone	Full name	Chemical structure	Iodine atoms
T₄	Thyroxine, tetraiodothyronine	3,5,3′,5′-tetraiodothyronine	4
T₃	Triiodothyronine	3,5,3′-triiodothyronine	3

Both are iodinated derivatives of the amino acid tyrosine.
Their basic structure consists of two iodinated benzene rings joined by an ether oxygen, with an alanine-like side chain containing NH₂ and COOH.
T₄ is the main hormone secreted by thyroid follicular cells and acts mainly as a prohormone.
T₃ is formed partly in the thyroid and largely by peripheral deiodination of T₄. It is the more biologically active hormone.
Removing iodine from the outer ring of T₄ produces active T₃. Removal from the inner ring produces inactive reverse T₃.

Ganong's Review of Medical Physiology, Twenty Sixth Edition, pp. 3656-3658.

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