リオチロニンナトリウム
リオチロニンナトリウムは、甲状腺ホルモンの活性細胞形態である**T3（トリヨードチロニン）**の合成形態です。 レボチロキシン（T4）が甲状腺機能低下症の標準的な第一選択薬ですが、リオチロニンはT4の補充に反応しない特定の症例に限定して使用されます。この無反応は、T4の消化管吸収不良、または理論的には脱ヨード酵素によるT4からT3への末梢変換の欠陥による可能性があります。 **臨床上のポイント：** * リオチロニンはレボチロキシンに比べて**作用時間と半減期が非常に短い**ため、より頻繁な投与（通常は1日3回）が必要です。 * 血清T3濃度のみを正常化し、T4レベルは低いままとなるため、正常な視床下部-下垂体-甲状腺のフィードバック軸を乱すことから、**初期治療には推奨されません**。 * 体の自然な調節変換ステップ（T4 → T3）をバイパスするため、医原性甲状腺中毒症のリスクが高くなります。
作用機序: Thyroid hormones act primarily at the cellular level to regulate metabolism. * **Mechanism:** T3 enters the cell and binds to **nuclear thyroid hormone receptors (TRs)** → the receptor-hormone complex binds to DNA → stimulates or inhibits the transcription of specific mRNA → alters protein synthesis. * **Physiologic Effects:** Increases the metabolic rate of tissues, enhances oxygen consumption, increases body temperature, stimulates gluconeogenesis, and promotes the mobilization and utilization of glycogen stores. * **Cardiovascular Effects:** Increases heart rate, cardiac output, and blood volume by upregulating beta-adrenergic receptors. * **Development:** Crucial for the adequate development of the central nervous system and skeletal growth.
動物種別の用量
- Hypothyroidism · 4.4 micrograms/kg PO 2-3 times a day · PO · q8h-q12h
- T3 suppression test (diagnostic for hyperthyroidism) · 25 μg · PO · q8h · 7 doses total · Administer 7 doses, then measure T4 and T3 on the morning of the 3rd day, 2-4 hours after the 7th dose.
- Hypothyroidism (unresponsive to T4) · 4-6 micrograms/kg PO q8h · PO · q8h · Some dogs may require less frequent dosing. Only indicated when T4 supplementation has failed.
- Hypothyroidism (refractory to T4) · 4-6 μg/kg · PO · q8h · Not recommended as first-line therapy due to short half-life.
用量は獣医療従事者向けの臨床リファレンスです。必ず最新の添付文書と個々の患者で確認してください。
投与経路
禁忌
- Acute myocardial infarction
- Thyrotoxicosis
- Untreated adrenal insufficiency
- Untreated adrenal insufficiency (Addison's disease)
有害事象
- Tachycardia
- Polyphagia
- Polyuria/Polydipsia (PU/PD)
- Excitability
- Nervousness
- Excessive panting
- Apathy or listlessness (seen in about 10% of cats)
- PU/PD (Polyuria/Polydipsia)
- Nervousness/Anxiety
- Panting
- Weight loss
薬物相互作用
- Tricyclic/Tetracyclic Antidepressants · Increased risk for CNS stimulation and cardiac arrhythmias.
- Antidiabetic Agents (insulin, oral agents) · Thyroid hormones may increase the requirements for insulin or oral antidiabetic agents.
- Cholestyramine · May reduce liothyronine absorption; separate doses by at least 4 hours.
- Sympathomimetic Agents (epinephrine, norepinephrine) · Thyroid hormones can potentiate the cardiovascular effects of sympathomimetics.
- Warfarin · Thyroid hormones increase the catabolism of vitamin K-dependent clotting factors, potentially increasing anticoagulation effects.
- Digoxin · Potential for reduced digoxin levels and decreased therapeutic effect.
- Ketamine · Concurrent use may cause tachycardia and hypertension. · moderate
- Sympathomimetics · Increased risk of cardiac arrhythmias and coronary insufficiency · moderate
- Insulin · May alter glucose control, requiring dosage adjustments · minor
モニタリング
- Serum T3 levels (draw just prior to dosing and 2-4 hours post-pill)
- Clinical signs of hyperthyroidism/thyrotoxicosis
- Note: Serum T4 levels will remain low during therapy
- Serum T3 levels
- Clinical signs of hyperthyroidism
- Heart rate and rhythm
過量投与
**Chronic Overdosage:** Produces signs of iatrogenic hyperthyroidism, including tachycardia, polyphagia, PU/PD, excitability, nervousness, and excessive panting. Some cats (approx. 10%) may exhibit 'apathetic' hyperthyroidism (listlessness, anorexia). **Treatment:** Reduce dosage or temporarily withhold until signs subside. **Acute Massive Overdosage:** Can produce signs resembling a **thyroid storm**. * **Treatment:** If oral ingestion is recent, reduce absorption using standard protocols (emetics, gastric lavage, activated charcoal, cathartics) unless contraindicated by the patient's clinical status. * **Supportive Care:** Oxygen, artificial ventilation, beta-blockers (e.g., propranolol) for tachycardia/arrhythmias, IV fluids, dextrose, and antipyretic agents as needed.
VetSheet の薬剤リファレンスは、獣医療従事者向けの臨床意思決定支援を目的としており、専門的判断やメーカーの最新添付文書に代わるものではありません。