Levothyroxine — Thyroid hormone / T4 replacement
Levothyroxine (synthetic L-thyroxine, T4) is the primary secretory product of the thyroid gland. T4 itself is largely inactive — it requires peripheral deiodination (by type 1 and type 2 deiodinase enzymes, DIO1/DIO2) to the active T3 (triiodothyronine) in liver, kidneys, and target tissues. T3 binds thyroid hormone receptors (TRα and TRβ) — nuclear receptors that regulate basal metabolic rate, cardiovascular function, neurological development, and thermogenesis. In hypothyroidism, insufficient T4/T3 leads to: fatigue, weight gain, bradycardia, constipation, cold intolerance, depression, hyperlipidemia, myopathy, and cognitive impairment. Levothyroxine monotherapy remains standard of care per ATA 2014, as peripheral T4→T3 conversion is highly efficient in most patients (though ~15% are poor DIO2 converters due to Thr92Ala polymorphism who may prefer T3 addition).
نظرة عامة
هذه الصفحة جزء من مكتبة المركّبات المُصنَّفة بالأدلة في Hormonaly. جميع الادعاءات السريرية مرتبطة بمصادر علمية مُحكَّمة عبر خط أنابيب التحقق المزدوج للاستشهادات.
فئة المركّب
Thyroid hormone / T4 replacement
آلية العمل
Levothyroxine (synthetic L-thyroxine, T4) is the primary secretory product of the thyroid gland. T4 itself is largely inactive — it requires peripheral deiodination (by type 1 and type 2 deiodinase enzymes, DIO1/DIO2) to the active T3 (triiodothyronine) in liver, kidneys, and target tissues. T3 binds thyroid hormone receptors (TRα and TRβ) — nuclear receptors that regulate basal metabolic rate, cardiovascular function, neurological development, and thermogenesis. In hypothyroidism, insufficient T4/T3 leads to: fatigue, weight gain, bradycardia, constipation, cold intolerance, depression, hyperlipidemia, myopathy, and cognitive impairment. Levothyroxine monotherapy remains standard of care per ATA 2014, as peripheral T4→T3 conversion is highly efficient in most patients (though ~15% are poor DIO2 converters due to Thr92Ala polymorphism who may prefer T3 addition).
الوضع التنظيمي
FDA-approved (NDA 021200 and others). Multiple branded (Synthroid, Tirosint, Levoxyl) and generic formulations. Note: brand-to-generic switches can affect TSH — maintain consistent formulation.
مستوى الأدلة
Very High — decades of clinical use, multiple large RCTs. ATA 2014 guidelines designate as standard of care. Consistently one of the most prescribed medications in the world.