Synthroid vs Levothyroxine: are they the same?
Synthroid is a commercial brand whose active ingredient is exactly Levothyroxine. This page explains what that means in practice: where they differ (brand, manufacturer, price, available strengths), why they are clinically equivalent, and when the brand or an authorised generic is the better choice.
Are they the same?
Yes — Synthroid contains Levothyroxine as its active ingredient. Talking about "Synthroid" in clinical terms is the same as talking about Levothyroxine at the dose strengths 25mcg, 50mcg, 75mcg, 100mcg, 150mcg. The differences from the authorised generic are brand, manufacturer (AbbVie), excipients, tablet appearance and price — not the active molecule itself.
When Synthroid is used
シンスロイドはあらゆる原因の甲状腺機能低下症(橋本甲状腺炎、甲状腺切除後、放射性ヨウ素治療後、先天性)、甲状腺腫、および分化型甲状腺癌後のTSH抑制に承認されています。現行ガイドラインによれば、レボチロキシン単独療法が甲状腺機能低下症の標準的な第一選択治療であり、T4/T3配合剤または乾燥甲状腺末は通常の第一選択治療として推奨されていません。
Mechanisms compared
Because they share the same active ingredient, the mechanism is identical: レボチロキシンは欠乏している内因性チロキシンを補充し、組織内で脱ヨード酵素により活性ホルモンであるトリヨードチロニン(T3)に変換されます。適切な補充により核内受容体での正常な甲状腺シグナル伝達が回復し、代謝率、心血管機能、成長および発達が正常化されます。治療目標は検査基準範囲内のTSH(および遊離T4)で評価される甲状腺機能正常状態の回復であり、約7日の長い半減期により1日1回投与で安定した血漿中濃度が得られます。
Brand or generic — which to choose?
For most patients, authorised generics of Levothyroxine are clinically equivalent substitutes for Synthroid and typically cost substantially less. The practical differences are manufacturer, excipients (rarely matters except in specific sensitivities), tablet appearance and price. The brand is preferred by some patients for personal reasons or insurance coverage; the generic is the default when cost matters.
よくある質問
Is Synthroid the same as Levothyroxine? ▾
Clinically yes — Synthroid is the brand and Levothyroxine is its active ingredient. At equivalent doses (25mcg, 50mcg, 75mcg, 100mcg, 150mcg) they produce the same effect. The difference is brand, manufacturer (AbbVie) and price.
Is generic Levothyroxine as effective as Synthroid? ▾
Yes. Authorised generics of Levothyroxine must demonstrate bioequivalence to the brand. The active substance is the same; average effect is the same. Price is usually substantially lower.
Why does Synthroid cost more than generic Levothyroxine? ▾
The price difference reflects originator development costs, brand recognition and competition among generic manufacturers. The molecule is identical; the cost is not.
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