CHP 6 (155)
THYROID GLAND AND PARATHYROID GLANDS
The thyroid gland is a butterfly-shaped structure located on the ventral surface of the trachea.
It secretes tetraiodothyronin (T )4and triiodothyronin (T ),3as well as calcitonin.
It regulates the body’s metabolic rate, and is necessary for normal growth and development.
Calcitonin regulates Ca levels in the blood.
The 4 parathyroid glands are smaller structures located on the posterior surface of the thyroid gland. They
secrete parathyroid hormone (PTH), an important regulator of Ca levels in the blood.
CHP 21 (625-627)
Thyroid hormones show little variation and their plasma levels are nearly steady. They don’t normally trigger
effects, instead they work to maintain status quo.
SYNTHESIS AND SECRETION OF THYROID HORMONES
The thyroid and gland contains numerous follicles that produce thyroid hormones.
Each consists of a single outer layer of follicular cells surrounding a central protein rich colloid secreted by the
In the interstitial space between the follicles are C cells, which synthesize and secrete calcitonin.
Thyroglobulin (TG), found in colloid, is a protein that functions as a precursor molecule for thyroid hormones.
Also has enzymes required for thyroid hormone synthesis and iodide.
The thyroglobulin and enzymes are synthesized in the follicular cells and secreted into the colloid by exocytosis.
From the blood, iodide is actively transported by follicular cells into colloids.
Steps of thyroid hormone synthesis and secretion:
1. Tyrosine residues of TG are iodinated. Addition of one iodide forms monoidotyrosine (MIT), whereas
addition of two forms diiodotyrosine (DIT).
2. 2 iodinated tyrosine residues (MIT, DIT) on the same TG molecule are coupled at which the 2 tyrosine
residues are linked together by a covalent bond. 2 DIT = T , DIT + MIT = T . T and T are thyroid
4 3 3 4
hormones, although they are still attached to TG. 2 MITs can’t combine.
3. Thyroid hormones are stored in the colloid bound to TG for up to 3 months.
4. Thyroid stimulating hormone (TSH) arriving via the blood-stream binds to receptors on the membrane
of the follicular cells, activating cAMP 2d messenger system. Results in phosphorylation of follicular cell
proteins to release thyroid hormones.
5. The follicular cells take in iodinated TG molecules form the colloid by phagocytosis.
6. The phagosome containing the iodinated TG fuses with a lysosome.
7. Exposure of the TG molecule to lysosomal enzymes that break down t