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Summary H77 - SV Guyton and Hall
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Samenvatting van H77 uit Guyton and Hall's Textbook of Medical Physiology.
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Guyton and HallCHAPTER 77 – Thyroid Metabolic Hormones
The thyroid secretes 2 major hormones:
Thyroxine (T4) 93%
Triiodothyronine (T3) 7%
The thyroid gland also secretes calcitonin, a hormone involved in calcium metabolism.
Thyroid secretion is controlled primarily by thyroid-stimulating hormone (TSH) secreted by
the anterior pituitary gland.
All the thyroxine is eventually converted to triiodothyronine in the tissues, so both are
functionally important. Triiodothyronine is about 4x more potent as thyroxine, but it is
present in the blood in much smaller quantities and has a shorter lifespan.
The thyroid gland is composed of follicles that are filled with colloid and lined with cuboidal
epithelial cells that secrete into the interior of the follicles. The major constituent of colloid
is the large glycoprotein thyroglobulin, which contains the thyroid hormones. The gland also
contains C cells that secrete calcitonin, a hormone that contributes to regulation of plasma
calcium ion concentration.
Iodine is required for the formation of thyroxine. The first step in this formation is the
transport of iodides from the blood into the thyroid glandular cells and follicles. This
pumping of the iodide to the interior of the cell is achieved by the action of a sodium-iodide
symporter (NIS). This process of concentrating the iodide in the cell is called iodide trapping.
The rate of iodide trapping is influenced by several factors, the most important being the
concentration of TSH; TSH stimulates the activity of the iodide pump. Iodide is transported
out of the thyroid cells across the apical membrane into the follicle by a chloride-iodide ion
counter-transporter called pendrin.
Thyroid hormones form within the thyroglobulin molecule. Thyroglobulin contains tyrosine
amino acids that combine with iodine to form the thyroid hormones. The thyroxine and
triiodothyronine hormones formed from the tyrosine amino acids remain part of the
thyroglobulin molecule during synthesis of the thyroid hormones and even afterward as
stored hormones in the follicular colloid.
Most of the thyroglobulin is not released into the circulating blood; instead, thyroxine and
triiodothyronine are cleaved from the thyroglobulin molecule, and then these free
hormones are released. 3 quarters of the iodinated tyrosine in the thyroglobulin never
become thyroid hormones but remain MIT and DIT. These are also freed from the
thyroglobulin molecules. However, they are not secreted into the blood, but their iodine is
cleaved from them to be used again for recycling.
The hormone finally delivered to and used by the tissues is mainly triiodothyronine.
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