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Levothyroxine
- indication:For use alone or in combination with antithyroid agents to treat hypothyroidism, goiter, chronic lymphocytic thyroiditis, myxedema coma, and stupor.
- pharmacologypharmacology:
- mechanism: Levothyroxine acts like the endogenous thyroid hormone thyroxine (T<sub>4</sub>, a tetra-iodinated tyrosine derivative). In the liver and kidney, T<sub>4</sub> is converted to T<sub>3</sub>, the active metabolite. In order to increase solubility, the thyroid hormones attach to thyroid hormone binding proteins, thyroxin-binding globulin, and thyroxin-binding prealbumin (transthyretin). Transport and binding to thyroid hormone receptors in the cytoplasm and nucleus then takes place. Thus by acting as a replacement for natural thyroxine, symptoms of thyroxine deficiency are relieved.
- toxicity: LD<sub>50</sub>=20 mg/kg (orally in rat). Hypermetabolic state indistinguishable from thyrotoxicosis of endogenous origin. Symptoms of thyrotoxicosis include weight loss, increased appetite, palpitations, nervousness, diarrhea, abdominal cramps, sweating, tachycardia, increased pulse and blood pressure, cardiac arrhythmias, tremors, insomnia, heat intolerance, fever, and menstrual irregularities.
- absorprion: Bioavailability varies from 48% to 80%. Human studies have confirmed the importance of an intact jejunum and ileum for levothyroxine absorption and have shown some absorption from the duodenum.
- halflife: T<sub>4</sub>, 6 to 7 days. T<sub>3</sub>, 1 to 2 days.
- roouteelimination: Thyroid hormones are primarily eliminated by the kidneys.
- volumedistribution:
- clearance: