左旋甲狀腺素鈉
左旋甲狀腺素鈉是甲狀腺分泌的內源性激素甲狀腺素 (T4) 的合成左旋異構體。它被認為是獸醫學中甲狀腺激素替代療法的**黃金標準**，主要用於治療犬隻的甲狀腺機能低下症，偶爾也用於其他物種。 **臨床重點：** * **前驅激素特性：** T4 主要是一種前驅激素，在周邊組織中會轉化為生物活性更強的 T3。 * **生物利用度：** 個體動物的口服吸收率差異很大，且可能受食物影響。通常建議空腹給藥以確保吸收一致。 * **治療監測：** 由於個體藥代動力學的差異，必須進行血清總 T4 (TT4) 的治療藥物監測 (TDM)，以針對個別病患調整劑量。
作用機制: 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.
各物種劑量
- 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.
給藥途徑
禁忌症
- Acute myocardial infarction
- Thyrotoxicosis
- Untreated adrenal insufficiency
不良反應
- Tachycardia
- Polyphagia
- Polyuria/Polydipsia (PU/PD)
- Excitability
- Nervousness
- Excessive panting
- Apathy or listlessness (seen in some cats)
藥物相互作用
- 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
監測
- 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
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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