How should thyroid replacement be initiated?
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Levothyroxine (LT4) should be used alone as initial replacement for patients with hypothyroidism (strength of recommendation [SOR]: A). The optimal initial dose is 1.6 μg/kg/d for healthy people aged 60 years or younger (SOR: B). Patients aged more than 60 years may require less levothyroxine to achieve therapeutic serum thyroid hormone replacement, so initial replacement should be decreased to 25 to 50 μg daily (SOR: C). Since patients with known heart disease may develop dysrhythmias, angina, and myocardial infarctions when started on full replacement doses, experts recommend starting 12.5 to 25 μg daily for this population (SOR: C). Brand-name (Synthroid, Levoxyl, etc) and generic LT4 are bioequivalent (SOR: B), although the US Food and Drug Administration (FDA) does not consider these products to be interchangeable until proven therapeutically equivalent.
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