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Related to l-thyroxine: hypothyroidism, L-tyrosine

levothyroxine sodium (L-thyroxine, T4)

Eltroxin (CA) (UK), Euthyrox (CA), Evotrox (UK), Levo-T, Levolet, Levothroid, Levoxyl, Nu-Thyro (CA), Soloxine, Synthroid, Unithroid

Pharmacologic class: Synthetic thyroxine hormone

Therapeutic class: Thyroid hormone replacement

Pregnancy risk category A

FDA Box Warning

• Drug shouldn't be used alone or with other agents to treat obesity or weight loss. In euthyroid patients, doses within range of daily hormonal requirements are ineffective for weight loss. Larger doses may cause serious or life-threatening toxicity, particularly when given with sympathomimetic amines (such as those used for anorectic effects).


Synthetic form of thyroxine that replaces endogenous thyroxine, increasing thyroid hormone levels. Thyroid hormones help regulate cell growth and differentiation and increase metabolism of lipids, protein, and carbohydrates.


Powder for injection: 200 mcg/vial in 6- and 10-ml vials, 500 mcg/vial in 6- and 10-ml vials

Tablets: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, 200 mcg, 300 mcg

Indications and dosages

Hypothyroidism; treatment or prevention of euthyroid goiter

Adults: For healthy adults younger than age 50 and those over age 50 who have recently been treated or undergone short-term therapy, start at full replacement dosage of 1.7 mcg/kg P.O. daily, given 30 minutes to 1 hour before breakfast. For patients older than age 50 or younger than age 50 with heart disease, 25 to 50 mcg P.O. daily, increased q 4 to 6 weeks. In severe hypothyroidism, initial dosage is 12.5 to 25 mcg P.O. daily, adjusted by 25 mcg daily q 2 to 4 weeks. For patients who can't tolerate oral doses, adjust I.M. or I.V. dosage to roughly half of oral dosage.

Congenital hypothyroidism

Children older than age 12 who have completed puberty and growth: 1.7 mcg/kg P.O. daily

Children older than age 12 who have not completed puberty and growth: Up to 150 mcg or 2 to 3 mcg/kg P.O. daily

Children ages 6 to 12: 4 to 5 mcg/kg P.O. daily

Children ages 1 to 5: 5 to 6 mcg/kg P.O. daily

Infants ages 6 to 12 months: 6 to 8 mcg/kg P.O. daily


thyroxine, thyroxin

a hormone of the thyroid gland that contains iodine and is a derivative of the amino acid tyrosine. The chemical name for thyroxine is tetraiodothyronine (symbol, T4); it is formed and stored in the thyroid follicles as thyroglobulin, the storage form. Thyroxine is released from the gland by the action of a proteolytic enzyme. T4 is deiodinated in peripheral tissues to form tri-iodothyronine (T3), which has a greater biological activity.
Thyroxine acts as a catalyst in the body and influences a great variety of effects, including metabolic rate (oxygen consumption); growth and development; metabolism of carbohydrates, fats, proteins, electrolytes and water; vitamin requirements; reproduction; and resistance to infection.
Thyroxine can be extracted from animals or made synthetically; it is used in the treatment of hypothyroidism and some types of goiter.

free thyroxine
the metabolically active fraction of thyroxine; abbreviated FT4, Tf. T4 = Tf + TBG. The amount is very small and difficult to estimate so that the amount present in serum is not used as a more accurate indicator of thyroxine status than T4.
thyroxine-binding globulin (TBG)
most (99.95%) of the thyroxine in plasma is bound to globulin and a small amount bound to prealbumin.
thyroxine-binding prealbumin
bound to a small fraction of circulating T4. This is the only known function of prealbumins.
References in periodicals archive ?
Further work-up failed to show any evidence for hyperparathyroidism, but these patients were all receiving L-thyroxine for various thyroid problems, and their thyrotropin (TSH) concentrations were very suppressed (<0.
Ferrous sulfate is known to inhibit absorption of L-thyroxine, as are several other medications including sucralfate, cholestyramine, colestipol, and aluminum-containing antacids.
Surks MI, Schadlow AR, stock JM, et al: Determination of iodothyronine absorption and conversion of L-thyroxine ([T.
Changes in bone mass during prolonged subclinical hyperthyroidism due to L-thyroxine treatment: a meta-analysis.
L-thyroxine augmentation of serotonergic antidepressants in female patients with refractory depression.
In addition, 17% of the PCOS group had clinical autoimmune thyroiditis: Eight patients were on L-thyroxine and two patients had already undergone surgery for Graves' disease.
5% reported thyroid disease, thyroid surgery, and/or use of thyroid medications (12 used L-thyroxine and 1 used thyrostatic drugs), and 61.
The stock solution containing 10 [micro]mol/L L-thyroxine was prepared as follows: 38.
Post-operatively she had low serum 8AM cortisol and Thyroxine (T4) and was put on L-thyroxine 100 [micro]g/day and prednisolone 5 mg daily.