レボチロキシンナトリウム
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.
動物種別の用量
- Hypothyroidism · 0.05-0.1 mg per cat PO once daily · PO · q24h · Wait a minimum of 4-6 weeks to assess clinical response.
- Post thyroidectomy · 0.1-0.2 mg (total dose) PO once daily beginning 24-48 hours post-op · PO · q24h · Several weeks or months · Monitor T4 levels to determine when supplementation can be ceased.
- Hypothyroidism · 10 mg in 70 mL of corn syrup once daily · PO · q24h · Monitor T4 levels one week after initiation of therapy.
- 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 · Taper off by administering 24 mg/day for 2 weeks and then 12 mg/day for 2 weeks.
- Hypothyroidism · One 0.1 mg tablet in 30 mL-120 mL of water daily · PO · q24h · Stir water and offer for 15 minutes and remove. Use high dose for budgerigars and low dose for water drinkers.
- 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 · Evaluate clinical response 4-8 weeks later and draw a T4 level 4-6 hours post dosing.
- Hypothyroidism · 10-22 micrograms/kg q12h or q24h · PO · q12h or q24h · Maximum of 0.8 mg q12h. Reevaluate 1-2 months after initiating therapy.
投与経路
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)
薬物相互作用
- Amiodarone · May decrease the metabolism of T4 to T3
- Antacids, Oral · May reduce levothyroxine absorption; separate doses by 4 hours
- Antidepressants, Tricyclic/Tetracyclic · Increased risk for CNS stimulation and cardiac arrhythmias
- Antidiabetic Agents (insulin, oral agents) · Levothyroxine may increase requirements for insulin or oral agents
- Cholestyramine · May reduce levothyroxine absorption; separate doses by 4 hours
- Corticosteroids (high dose) · Decreased conversion of T4 to T3
- Digoxin · Potential for reduced digoxin levels
- Ferrous Sulfate · May reduce levothyroxine absorption; separate doses by 4 hours
- Ketamine · May cause tachycardia and hypertension
- Phenobarbital · Possible increased metabolism of thyroxine; dosage adjustments may be needed
- Propylthiouracil · Decreased conversion of T4 to T3
- Rifampin · Possible increased metabolism of thyroxine; dosage adjustments may be needed
- Sertraline · May increase levothyroxine requirements
モニタリング
- 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
過量投与
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).
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