Thyroxine

# Thyroxine ## Overview - **Classification**: Synthetic Thyroid Hormone (L-Thyroxine, T4) - **Mechanism**: Replaces endogenous thyroid hormone, converted to triiodothyronine (T3) in peripheral tissues. T3 enters cells and binds to nuclear thyroid hormone receptors, regulating gene expression and metabolism. ## Primary Indications 1. **Hypothyroidism**: Replacement therapy for all etiologies (e.g., Hashimoto's, post-thyroidectomy). 2. **TSH Suppression**: Adjunct to surgery and radioiodine in the management of thyroid-dependent tumors (e.g., thyroid cancer). 3. **Goiter**: Treatment of diffuse non-toxic goiter. ## Adult Dosing ### Standard Dosing **Hypothyroidism (Adults)** - **Dose**: Initial **1.6 mcg/kg/day** (based on ideal body weight). - **Frequency**: Once daily. - **Route**: Oral. - **Duration**: Lifelong, typically. - **Special Considerations**: Titrate dose by **12.5-25 mcg** increments every 4-6 weeks based on TSH and clinical response. **TSH Suppression (Thyroid Cancer)** - **Dose**: Individualized to achieve target TSH level (e.g., < **0.1 mIU/L** for high-risk). Often **2-2.5 mcg/kg/day**. - **Frequency**: Once daily. - **Route**: Oral. ### Dose Adjustments - **Renal Impairment**: No specific dose adjustment needed. - **Hepatic Impairment**: No specific dose adjustment needed. - **Elderly Patients**: Start with lower doses (**12.5-25 mcg/day**) due to increased cardiac sensitivity. Titrate slowly. ## Pediatric Dosing ### Neonates (0-28 days) - Congenital Hypothyroidism - **Dose**: **10-15 mcg/kg/day**. - **Frequency**: Once daily. - **Route**: Oral. - **Maximum**: No specific max/kg, adjust based on TSH/T4 response. - **Special Notes**: Crush tablet, mix with small amount of water/breast milk/formula. Administer immediately via spoon/syringe. Avoid mixing with soy formula. ### Infants (1-12 months) - Congenital/Acquired Hypothyroidism - **Dose**: **6-8 mcg/kg/day**. - **Frequency**: Once daily. - **Route**: Oral. - **Maximum**: No specific max/kg, adjust based on TSH/T4 response. ### Children (1-12 years) - Acquired Hypothyroidism - **Dose**: - 1-5 years: **5-6 mcg/kg/day**. - 6-12 years: **4-5 mcg/kg/day**. - **Frequency**: Once daily. - **Route**: Oral. - **Maximum**: Up to **150-200 mcg/day** in older children if required. ### Adolescents (13-18 years) - Acquired Hypothyroidism - **Dose**: Typically approaches adult dosing of **1.6 mcg/kg/day**. - **Frequency**: Once daily. - **Route**: Oral. - **Maximum**: Generally up to **200 mcg/day**, but higher doses may be needed for TSH suppression. ## Safety Information ### Contraindications - **Absolute**: Untreated thyrotoxicosis. - **Absolute**: Acute myocardial infarction. - **Absolute**: Untreated adrenal cortical insufficiency. ### Common Adverse Effects (Dose-related, mimic hyperthyroidism) - **Very Common (>10%)**: Palpitations, tachycardia, anxiety, insomnia, weight loss, tremor, sweating, heat intolerance. - **Common (1-10%)**: Headache, diarrhea, muscle weakness, menstrual irregularities. - **Serious but Rare**: Angina pectoris, arrhythmias, heart failure, osteoporosis (with long-term suppressive doses). ### Key Drug Interactions - **Iron, Calcium, Antacids (aluminum/magnesium), Sucralfate**: Impaired absorption of levothyroxine. Separate by **4 hours**. - **Proton Pump Inhibitors (PPIs), H2-blockers**: May reduce levothyroxine absorption. Monitor TSH. - **Cholestyramine, Colestipol**: Binds levothyroxine. Administer levothyroxine **4-5 hours prior**. - **Warfarin**: May enhance anticoagulant effect. Monitor INR, adjust warfarin dose. - **Estrogens (oral)**: Increase TBG, potentially requiring higher levothyroxine dose. - **Antiepileptics (e.g., Carbamazepine, Phenytoin)**: May increase levothyroxine metabolism. May require dose increase. ## Monitoring & Follow-up - **Before Treatment**: Baseline TSH, Free T4. Assess cardiac status. - **During Treatment**: - **Initial**: TSH, Free T4 every **4-6 weeks** until stable. - **Maintenance**: TSH, Free T4 every **6-12 months** or if symptoms change. - **Clinical Signs**: Monitor for symptoms of hypo- or hyperthyroidism (e.g., fatigue, weight changes, palpitations, anxiety). ## Clinical Pearls - 💡 **Timing**: Administer **once daily in the morning**, at least **30-60 minutes before breakfast** on an empty stomach for consistent absorption. - 💡 **Consistency**: Patients should take levothyroxine at the same time each day. Advise against switching brands due to potential bioavailability differences. - 💡 **Interactions**: Educate patients to separate doses from calcium, iron, antacids, and high-fiber foods. - 💡 **Onset**: Full therapeutic effect may take several weeks. Counsel patients on expected symptom improvement. - 💡 **Pediatric Administration**: For infants, crush tablets and mix with a small amount of water or breast milk/formula (not a full bottle) and administer immediately. > **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.