레보티록신 나트륨

갑상선 호르몬

레보티록신 나트륨은 갑상선에서 분비되는 내인성 호르몬인 티록신(T4)의 합성 좌선성 이성질체입니다. 수의학에서 갑상선 호르몬 대체 요법의 **표준 치료제(gold standard)**로 간주되며, 주로 반려견의 갑상선 기능 저하증 치료에 사용되고 때때로 다른 종에도 사용됩니다. **핵심 임상 정보:** * **전구호르몬 특성:** T4는 주로 전구호르몬으로 작용하며 말초 조직에서 생물학적 활성이 더 높은 T3로 전환됩니다. * **생체이용률:** 경구 흡수율은 개체마다 차이가 클 수 있으며 음식의 영향을 받을 수 있습니다. 일관된 흡수를 위해 공복 투여가 권장되는 경우가 많습니다. * **치료 모니터링:** 개체 간 약동학적 다양성 때문에 개별 환자에게 용량을 맞추기 위해 혈청 총 T4(TT4)의 치료 약물 모니터링(TDM)이 필수적입니다.

작용 기전: 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.

동물 종별 용량

Cats

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.

Horses

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.

Birds

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.

Dogs

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.

투여 경로

POIV

금기

Acute myocardial infarction
Thyrotoxicosis
Untreated adrenal insufficiency

이상반응

Tachycardia
Polyphagia
Polyuria/Polydipsia (PU/PD)
Excitability
Nervousness
Excessive panting
Apathy or listlessness (seen in some cats)

약물 상호작용

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

모니터링

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

과용량

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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