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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]
By the 1960s, it was known that thyroxine was the essential hormone produced by the thyroid gland, and that most T3 was manufactured in other parts of the body by deiodination of thyroxine. It was demonstrated in hypothyroid animals and people that replacement of thyroxine alone corrected the measurable manifestations (laboratory test results ...
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 ...
The major form of thyroid hormone in the blood is thyroxine (T 4), whose half-life of around one week [4] is longer than that of T 3. [5] In humans, the ratio of T 4 to T 3 released into the blood is approximately 14:1. [6] T 4 is converted to the active T 3 (three to four times more potent than T 4) within cells by deiodinases (5′-deiodinase).
Blood free thyroxine and TSH levels are monitored to help determine whether the dose is adequate. This is done 4–8 weeks after the start of treatment or a change in levothyroxine dose. Once the adequate replacement dose has been established, the tests can be repeated after 6 and then 12 months, unless there is a change in symptoms. [8]
Three related isoforms, deiodinase type I, II, and III, contribute to activation and inactivation of the initially released hormone precursor T 4 into T 3 (triiodothyronine) or rT 3 (reverse triiodothyronine) in target cells. The enzymes catalyze a reductive elimination of iodine (the different isoforms attack different thyronine positions ...
T 3 is the more metabolically active hormone produced from T 4.T 4 is deiodinated by three deiodinase enzymes to produce the more-active triiodothyronine: . Type I present in liver, kidney, thyroid, and (to a lesser extent) pituitary; it accounts for 80% of the deiodination of T 4.
Hypothyroidism is common in pregnancy with an estimated prevalence of 2-3% and 0.3-0.5% for subclinical and overt hypothyroidism respectively. [8] Endemic iodine deficiency accounts for most hypothyroidism in pregnant women worldwide while chronic autoimmune thyroiditis is the most common cause of hypothyroidism in iodine sufficient parts of the world.