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Levothyroxine is the chemical name of the manufactured version of T 4, which is metabolised more slowly than T 3 and hence usually only needs once-daily administration. Natural desiccated thyroid hormones are derived from pig thyroid glands, and are a "natural" hypothyroid treatment containing 20% T 3 and traces of T 2 , T 1 and calcitonin .
At the cellular level, T 3 is the body's more active and potent thyroid hormone. [2] T 3 helps deliver oxygen and energy to all of the body's cells, its effects on target tissues being roughly four times more potent than those of T 4. [2] Of the thyroid hormone that is produced, just about 20% is T 3, whereas 80% is produced as T 4.
Thyroid follicular cells (also called thyroid epithelial cells or thyrocytes [1]) are the major cell type in the thyroid gland, and are responsible for the production and secretion of the thyroid hormones thyroxine (T 4) and triiodothyronine (T 3).
Low-T3 syndrome and high-T3 syndrome: Consequences of step-up hypodeiodination, e.g. in critical illness as an example for type 1 allostasis, [20] or hyperdeiodination, as in type 2 allostasis, including posttraumatic stress disorder. [12] Resistance to thyroid hormone: Feedback loop interrupted on the level of pituitary thyroid hormone receptors.
When circulating in the body, T3 and T4 are bound to transport proteins. Only a small fraction of the circulating thyroid hormones are unbound or free, and thus biologically active. T3 and T4 levels can thus be measured as free T3 and T4, or total T3 and T4, which takes into consideration the free hormones in addition to the protein-bound hormones.
Thyroid-stimulating hormone (also known as thyrotropin, thyrotropic hormone, or abbreviated TSH) is a pituitary hormone that stimulates the thyroid gland to produce thyroxine (T 4), and then triiodothyronine (T 3) which stimulates the metabolism of almost every tissue in the body. [1]
[2] Total T4 is measured to see the bound and unbound levels of T4. The total T4 is less useful in cases where there could be protein abnormalities. The total T4 is less accurate due to the large amount of T4 that is bound. The total T3 is measured in clinical practice since the T3 has decreased amount that is bound as compared to T4. [citation ...
Besides its increased function in producing extrathyroid T 3 in patients with hyperthyroidism, its function is less well understood than D2 or D3 [2] [7] Deiodinase 2, located in the ER membrane, converts T 4 into T 3 and is a major source of the cytoplasmic T 3 pool. [2] Deiodinase 3 prevents T 4 activation and inactivates T 3. [9]