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Levothyroxine is a synthetic form of thyroxine (T 4), which is secreted by the thyroid gland. Levothyroxine and thyroxine are chemically identical: natural thyroxine is also in the "levo" chiral form, the difference is only in terminological preference. T 4 is biosynthesized from tyrosine. Approximately 5% of the US population suffers from over ...
As a measure of deiodination, the production of radioactive iodine and other physiological metabolites, in particular T 3 or reverse T 3, are determined and expressed (e.g. as fmol/mg protein/minute). [24] [25] In vivo, deiodination activity is estimated from equilibrium levels of free T 3 and free T 4.
In starvation or severe somatic stress, deiodinase type 1 is inhibited which lowers circulating levels of T 3 (due to it being the main source of peripherally converted T 3 from T 4 in the plasma), causing a decrease in the metabolic rate. [3]
Levothyroxine, a drug used to treat hypothyroidism, can lead to reduced bone mass and density in older adults with normal thyroid levels, a small cohort study has shown.
Thyroid hormones act on nearly every cell in the body. They act to increase the basal metabolic rate, affect protein synthesis, help regulate long bone growth (synergy with growth hormone) and neural maturation, and increase the body's sensitivity to catecholamines (such as adrenaline) by permissiveness. [12]
They recommend levothyroxine as the preferred treatment. Some practitioners refuse to use desiccated thyroid. [9] Each 64.8 mg (one grain) of thyroid extract contains approximately 38 μg and 9 μg of measurable levothyroxine (T4) and liothyronine (T3), respectively. [1] Arguments against desiccated thyroid include:
Thyrotoxicosis: Over-supply with thyroid hormones, e.g. by overdosed exogenously levothyroxine supplementation. 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 ...
The sum activity of peripheral deiodinases (G D, also referred to as deiodination capacity, total deiodinase activity or, if calculated from levels of thyroid hormones, as SPINA-GD [a]) is the maximum amount of triiodothyronine produced per time-unit under conditions of substrate saturation. [1]