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Chapter

PSL201Y1 Chapter Notes -Basal Metabolic Rate, Sympathetic Nervous System, Gluconeogenesis


Department
Physiology
Course Code
PSL201Y1
Professor
Yue Li

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THYROID HORMONES
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 (T4) and triiodothyronin (T3), as 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
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
follicular cells.
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 = T4, DIT + MIT = T3. T3 and T4 are thyroid
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 2nd 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 the thyroglobulin causes the
release of free T3 and T4 into the follicular cell. T3 and T4 are hydrophobic so they can diffuse across the
plasma membrane and into the bloodstream. They become selectively bound by protein carriers that
include thyroxine-binding globulin and transthyretin, and nonseletively bound by albumin.
T4 is normally produced and secreted about 10x more than T3. T3 is approx. 4x more potent at the target tissues.
T4 is converted y the liver, kidneys, or target tissues to the more active T3 (activation).
Thyroid hormone levels are virtually constant because the primary control of its secretion occurs via negative
feedback.
Secretion of TSH in the anterior pituitary is stimulated by thyrotropin releasing hormone (TRH) in the
hypothalamus. Once they are in the bloodstream, they limit the anterior pituitary and hypothalamus to secrete
more TRH and TSH.
T4 provides a stronger negative feedback than T3.
ACTIONS OF THYROID HORMONES
Thyroid hormones are hydrophobic and thus easily cross membranes, and the receptors for them are in the
nuclei of target cells.
Thyroid hormones + receptors alters the rate of transcription of mRNA, thereby altering protein synthesis.
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