Lifelong treatment with LT4, guided by levels of thyrotropin, is the mainstay of management of hypothyroidism. The bioavailability of LT4 is about 70% following an oral dose, with absorption occurring mainly in the ileum and jejunum. Maximum plasma concentrations of LT4 are achieved about 3 h after an oral dose in patients with hypothyroidism. The long terminal half-life of orally administered LT4, about 7.5 days, is consistent with once-daily dosing. Pregnancy, several medical conditions (especially) those affecting the gut, and a number of drugs, supplements, or foodstuffs can reduce the absorption and absolute bioavailability of LT4, or can alter the secretion of TSH, with detrimental consequences for long-term control of thyroid function. Poor adherence to LT4 therapy is also a common challenge. The introduction of novel formulations of LT4, with more precise delivery of the active ingredient and higher levels of bioequivalence with existing products will facilitate accurate titration of LT4 for patients with hypothyroidism.
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