レボチロキシンナトリウム
Levothyroxine Sodium
3,5,3',5'-tetra-iodo-L-thyronine sodium
別名: Soloxine · Synthroid · Levosyn · Thyro-Tabs · Thyrosyn · Leventa · Thyro-L · Levothroid · Levoxyl · Tirosint · T4 thyroxine sodium · levothyroxin natrium · levothyroxinum natricum · L-thyroxine sodium · tirossina · tiroxina sodica
VetSheet 獣医チームが監修更新日 2026年4月5日エビデンスに基づく獣医学リファレンス
用量は獣医専門家向けの臨床参考資料です。必ず最新の医薬品情報および個別症例に基づいて確認してください。
動物種別用量
🌎 NA — North America🌍 EU — Europe
犬
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Hypothyroidism | 20 micrograms/kg (0.02 mg/kg) body weight PO twice daily with a maximum dose of 0.8 mg twice daily | PO | q12h | — | 🌎 NA |
| Hypothyroidism | 10-22 micrograms/kg q12h or q24h | PO | q12h or q24h | — | 🌎 NA |
| Hypothyroidism | 0.02 mg/kg PO twice daily to start; (0.02-0.04 mg/kg PO once daily or, if necessary divided twice daily to maintain). Alternatively, give 0.5 mg/m2 | PO | q12h or q24h | — | 🌎 NA |
| Myxedema coma | 5 micrograms/kg IV q12h initially | IV | q12h | — | 🌎 NA |
- Hypothyroidism: Evaluate clinical response 4-8 weeks later and draw a T4 level 4-6 hours post dosing.
- Hypothyroidism: Maximum of 0.8 mg q12h. Reevaluate 1-2 months after initiating therapy.
- Hypothyroidism: 0.5 mg/m2 dosing may prevent hyperthyroid effects, particularly in large breed dogs.
- Myxedema coma: Oral administration may be poorly absorbed in this state.
猫
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Hypothyroidism | 0.05-0.1 mg per cat PO once daily | PO | q24h | — | 🌎 NA |
| Post thyroidectomy | 0.1-0.2 mg (total dose) PO once daily beginning 24-48 hours post-op | PO | q24h | Several weeks or months | 🌎 NA |
- Hypothyroidism: Wait a minimum of 4-6 weeks to assess clinical response.
- Post thyroidectomy: Monitor T4 levels to determine when supplementation can be ceased.
鳥
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Hypothyroidism | One 0.1 mg tablet in 30 mL-120 mL of water daily | PO | q24h | — | 🌎 NA |
- Hypothyroidism: Stir water and offer for 15 minutes and remove. Use high dose for budgerigars and low dose for water drinkers.
爬虫類
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Hypothyroidism in tortoises | 0.02 mg/kg PO every other day | PO | q48h | — | 🌎 NA |
馬
| 適応 | 用量 | 経路 | 頻度 | 期間 | 地域 |
|---|---|---|---|---|---|
| Hypothyroidism | 10 mg in 70 mL of corn syrup once daily | PO | q24h | — | 🌎 NA |
| Adjunctive treatment of equine metabolic syndrome | 0.1 mg/kg PO once daily, initiate treatment at 48 mg/day... and then increase to 72 mg/day after 3 months if the horse remains obese | PO | q24h | 6 months | 🌎 NA |
- Hypothyroidism: Monitor T4 levels one week after initiation of therapy.
- Adjunctive treatment of equine metabolic syndrome: Taper off by administering 24 mg/day for 2 weeks and then 12 mg/day for 2 weeks.
用量は獣医専門家向けの臨床参考資料です。必ず最新の医薬品情報および個別症例に基づいて確認してください。
概要
レボチロキシンナトリウムは、甲状腺から分泌される内因性ホルモンであるチロキシン（T4）の合成レボ異性体です。獣医学における甲状腺ホルモン補充療法のゴールドスタンダードとされており、主に犬の甲状腺機能低下症の治療に使用され、他の動物種にも使用されることがあります。
臨床上の重要ポイント:
- プロホルモンとしての性質: T4は主にプロホルモンであり、末梢組織でより生物活性の高いT3に変換されます。
- バイオアベイラビリティ: 経口吸収は個体差が大きく、食事の影響を受ける可能性があります。安定した吸収を得るため、絶食時の投与が推奨されることが多いです。
- 治療モニタリング: 個体ごとの薬物動態のばらつきがあるため、個々の患者に合わせて用量を調整するために、血清総T4（TT4）の治療薬物モニタリング（TDM）が不可欠です。
作用機序
Thyroid hormones regulate multiple metabolic processes, including fat, protein, and carbohydrate metabolism, and are critical for normal growth, development, and basal metabolic rate.
- Pathway: Exogenous levothyroxine (T4) is absorbed and distributed to peripheral tissues.
- Conversion: Inside cells, T4 is enzymatically deiodinated by 5'-deiodinase to triiodothyronine (T3), the primary active hormone.
- Receptor Binding: T3 enters the nucleus and binds to thyroid hormone receptors (TRs).
- Gene Transcription: The TR-T3 complex binds to thyroid response elements (TREs) on DNA → alters gene transcription → increases mRNA synthesis → increases protein synthesis, oxygen consumption, heart rate, and cellular metabolism.
安全性・警告
禁忌
- Acute myocardial infarction
- Thyrotoxicosis
- Untreated adrenal insufficiency
有害事象
- Tachycardia
- Polyphagia
- Polyuria/Polydipsia (PU/PD)
- Excitability
- Nervousness
- Excessive panting
- Apathy or listlessness (seen in some cats)
注意事項
Use with caution and at a lower initial dosage in patients with concurrent hypoadrenocorticism (treated), cardiac disease, diabetes, or in elderly patients. High fiber diets may reduce absorption. Hypothyroid dogs can have decreased GFR; restoration to a euthyroid state can increase GFR and reduce serum creatinine levels. Numerous drugs can affect thyroid function tests (T4, T3, TSH), so evaluate lab results accordingly.
医薬品相互作用
May decrease the metabolism of T4 to T3
May reduce levothyroxine absorption; separate doses by 4 hours
Increased risk for CNS stimulation and cardiac arrhythmias
Levothyroxine may increase requirements for insulin or oral agents
May reduce levothyroxine absorption; separate doses by 4 hours
Decreased conversion of T4 to T3
Potential for reduced digoxin levels
May reduce levothyroxine absorption; separate doses by 4 hours
May cause tachycardia and hypertension
Possible increased metabolism of thyroxine; dosage adjustments may be needed
Decreased conversion of T4 to T3
Possible increased metabolism of thyroxine; dosage adjustments may be needed
May increase levothyroxine requirements
May reduce levothyroxine absorption; separate doses by 4 hours
Levothyroxine can potentiate effects
Thyroid hormones increase the catabolism of vitamin K-dependent clotting factors that may increase the anticoagulation effects
監視
- Clinical signs of resolution of hypothyroidism (e.g., improved energy, hair regrowth, weight loss)
- Signs of thyrotoxicosis/hyperthyroidism (tachycardia, panting, weight loss with polyphagia)
- Serum total T4 (TT4) levels (typically measured 4-6 hours post-pill for peak levels)
- Canine TSH (cTSH) if indicated
薬物動態
半減期
吸収
Wide variability from animal to animal.
代謝
Converted in peripheral tissues from T4 to T3.
過剰投与
Chronic overdosage will produce signs of hyperthyroidism, including tachycardia, polyphagia, PU/PD, excitability, nervousness, and excessive panting. Some cats (approx. 10%) may exhibit 'apathetic' hyperthyroidism (listlessness, anorexia).
- Management of Chronic OD: Reduce dosage and/or temporarily withhold until signs subside.
- Acute Overdose: Less likely to cause severe thyrotoxicosis than chronic OD. Signs may include vomiting, diarrhea, hyperactivity to lethargy, hypertension, tachycardia, tachypnea, dyspnea, and abnormal pupillary light reflexes. In dogs, signs appear within 1-9 hours.
- Treatment of Acute OD: If ingestion occurred within 2 hours, use standard decontamination protocols (emetics, cathartics, activated charcoal). Provide supportive care (oxygen, artificial ventilation, beta-blockers like propranolol, fluids, dextrose, antipyretics).
製品
製剤
- Chewable tablets
- Oral solution
- Injection (lyophilized powder)
- Liquid-filled oral capsules
動物用医薬品
- Levothyroxine Sodium Tablets: 0.1 mg, 0.2 mg, 0.3 mg, 0.4 mg, 0.5 mg, 0.6 mg, 0.7 mg, 0.8 mg, 1 mg (Soloxine, Levosyn, Thyro-Tabs, Thyrosyn, Thyroxine-L, Thyrozine, Thyrokare)
- Levothyroxine Sodium Tablets Chewable: 0.1 mg to 0.8 mg (Canine Thyroid Chewable Tablets, Nutrived T-4, Heska Thyromed)
- Levothyroxine Oral Solution: 1 mg/mL (Leventa)
- Levothyroxine Sodium Powder: 0.22% (Equine Thyroid Supplement, Thyrozine Powder, Levoxine Powder, Thyro-L, Throxine-L Powder, Equi-Phar Thyrosyn Powder, Thyrokare Powder)
ヒト用医薬品
- Levothyroxine Sodium Tablets: 0.025 mg to 0.3 mg (Synthroid, Levothroid, Levoxyl, Unithroid)
- Levothyroxine Sodium Liquid-filled Oral Capsules: 13 mcg to 150 mcg (Tirosint)
- Levothyroxine Powder for Injection lyophilized: 200 mcg & 500 mcg
規制ステータス
規制データなし: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
保管・安定性
Store at room temperature in tight, light-resistant containers. The injectable product should be reconstituted immediately before use; discard unused portions. Do not mix injection with other drugs or IV fluids. Aqueous solutions are reportedly unstable.
飼主向け情報
重要: 継続が鍵です！食事は薬の吸収量に影響を与える可能性があるため、毎日同じ時間に、理想的には空腹時または常に同じ種類の食事と一緒にこの薬を与えてください。
- 生涯にわたる治療: 甲状腺機能低下症は通常、生涯にわたる病気です。獣医師に相談せずに薬の投与を中止しないでください。
- モニタリング: 用量が正確であることを確認するために、ペットは定期的な血液検査を受ける必要があります。少なすぎると症状は改善せず、多すぎると副作用を引き起こす可能性があります。
- 過剰投与の兆候に注意: ペットが過剰に水を飲んだり尿をしたりする、激しくパンティング（あえぎ呼吸）をする、神経質または活動的すぎる、あるいは食欲旺盛なのに体重が減少するなどの症状が見られた場合は、獣医師に連絡してください。
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