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Levothyroxine, also known as L-thyroxine, is a synthetic form of the thyroid hormone thyroxine (T 4). [5] [8] It is used to treat thyroid hormone deficiency (hypothyroidism), including a severe form known as myxedema coma. [5]
After total thyroidectomy or high-dose radioiodine therapy (e.g. in treated thyroid cancer) as well as after initiation of substitution therapy with levothyroxine the activity of step-up deiodinases decreases [23] [24] and the correlation of SPINA-GD to thyrotropin concentration is lost. [25]
The procedure is to take the child's weight in pounds, divide by 150 lb, and multiply the fractional result by the adult dose to find the equivalent child dosage.For example, if an adult dose of medication calls for 30 mg and the child weighs 30 lb, divide the weight by 150 (30/150) to obtain 1/5 and multiply 1/5 times 30 mg to get 6 mg.
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]
The first recorded American use dates to 1891 by a woman who was still taking it 52 years later at 84 years of age [18] Desiccated thyroid extract is prepared from pig thyroid glands. The glands are dried (desiccated), ground to powder, combined with binder chemicals, and pressed into pills.
If the medication is mixed with formulas containing iron or soya products, larger doses may be required, as these substances may alter the absorption of thyroid hormone from the gut. [16] Monitoring TSH blood levels every 2–3 weeks during the first months of life is recommended to ensure that affected infants are at the high end of normal range.
In the US, hypothyroidism and hyperthyroidism were respectively found in 4.6 and 1.3% of the >12y old population (2002). [3] In some types, such as subacute thyroiditis or postpartum thyroiditis, symptoms may go away after a few months and laboratory tests may return to normal. [4] However most types of thyroid disease do not resolve on their own.
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.