Levothyroxin

# Levothyroxin ## Overview - **Classification**: Thyroid hormone replacement, synthetic T4 - **Mechanism**: Replaces deficient endogenous thyroid hormone, converting to active triiodothyronine (T3) in peripheral tissues, regulating metabolism and growth. ## Primary Indications 1. **Hypothyroidism**: Treatment of decreased thyroid function of any etiology. 2. **TSH suppression**: In differentiated thyroid cancer (e.g., papillary, follicular) post-thyroidectomy. 3. **Myxedema Coma**: Severe, life-threatening hypothyroidism. ## Adult Dosing ### Standard Dosing **Hypothyroidism (Primary/Secondary)** - **Initial Dose**: **1.6-1.7 mcg/kg/day** (ideal body weight) or **12.5-50 mcg once daily**. - **Lower Initial Dose**: **12.5-25 mcg once daily** for elderly or those with cardiovascular disease. - **Maintenance Dose**: Typically **100-125 mcg once daily** (range **25-300 mcg/day**). - **Frequency**: Once daily - **Route**: Oral - **Special Considerations**: Adjust dose by **12.5-25 mcg** increments every **4-6 weeks** based on TSH. **TSH Suppression (Differentiated Thyroid Cancer)** - **Dose**: Typically **2.2-2.8 mcg/kg/day**, adjusted to achieve target TSH. - **Frequency**: Once daily - **Route**: Oral - **Special Considerations**: Target TSH is often **<0.1 mIU/L** or **0.1-0.5 mIU/L**, depending on risk. **Myxedema Coma (Emergency)** - **Dose**: **300-500 mcg** loading dose. - **Frequency**: Single loading dose, then **75-100 mcg once daily**. - **Route**: Intravenous (IV) - **Special Considerations**: Concurrent corticosteroid therapy is recommended. ### Dose Adjustments - **Renal Impairment**: No specific dose adjustment needed. Thyroid hormone elimination is largely hepatic. - **Hepatic Impairment**: No specific dose adjustment needed. Monitor thyroid function closely. - **Elderly Patients**: Start with lower doses (**12.5-25 mcg once daily**). Titrate slowly due to increased risk of cardiac events. ## Pediatric Dosing ### Neonates (0-28 days) - **Indication**: Congenital Hypothyroidism - **Dose**: **10-15 mcg/kg/day**. - **Frequency**: Once daily - **Route**: Oral - **Maximum**: Not clearly defined per dose, but based on TSH response. - **Special Notes**: Crucial for neurodevelopment. Crush tablet and mix with small amount of water/breast milk; give immediately. Avoid soy formula for 4 hours before/after. ### Infants (1-12 months) - **Indication**: Congenital or Acquired Hypothyroidism - **Dose**: **6-8 mcg/kg/day**. - **Frequency**: Once daily - **Route**: Oral - **Maximum**: Up to **50 mcg/day**. - **Special Notes**: Crush tablet and mix with small amount of liquid. Monitor TSH/fT4 frequently (every 1-3 months) for growth and development. ### Children (1-12 years) - **Indication**: Acquired Hypothyroidism - **Dose**: **4-6 mcg/kg/day** for ages 1-5 years; **3-5 mcg/kg/day** for ages 6-12 years. - **Frequency**: Once daily - **Route**: Oral - **Maximum**: Typically up to **100-150 mcg/day** (or adult equivalent). - **Special Notes**: Doses may be slightly higher if persistent symptoms or TSH is elevated. Ensure compliance. ### Adolescents (13-18 years) - **Indication**: Acquired Hypothyroidism - **Dose**: **2-4 mcg/kg/day**, adjusting towards adult maintenance doses (**1.6-1.7 mcg/kg/day**). - **Frequency**: Once daily - **Route**: Oral - **Maximum**: Adult maintenance dose (typically **100-125 mcg/day**). - **Special Notes**: Growth spurts may necessitate dose increases. Monitor for adherence, TSH goal is important for puberty. ## Safety Information ### Contraindications - **Absolute**: Acute myocardial infarction (MI) - **Absolute**: Untreated adrenal cortical insufficiency ("Addison's disease") - **Absolute**: Untreated thyrotoxicosis (overactive thyroid) - **Relative**: Hypersensitivity to levothyroxine or excipients. ### Common Adverse Effects - **Very Common (>10%)**: Usually signs of overtreatment (hyperthyroidism). - **Common (1-10%)**: Palpitations, tachycardia, arrhythmias, tremors, headache, nervousness. - **Common (1-10%)**: Diarrhea, abdominal cramps, weight loss, increased appetite, heat intolerance. - **Serious but Rare**: Cardiovascular events (angina, MI), osteoporosis (long-term overdose), pseudotumor cerebri (children). ### Key Drug Interactions - **Antacids (Aluminum/Magnesium), Iron, Calcium, Sucralfate**: Form insoluble complexes; separate administration by **4 hours**. - **Proton Pump Inhibitors (PPIs) / H2 Blockers**: Decrease gastric acid, potentially impairing absorption; monitor TSH. - **Warfarin**: May enhance anticoagulant effect; monitor INR/PT closely, reduce warfarin dose if needed. - **Antidiabetics (Insulin, Oral Hypoglycemics)**: May increase antidiabetic requirements; monitor blood glucose. - **Cholestyramine/Colestipol, Kayexalate**: Bind levothyroxine in gut; separate administration by **4 hours**. - **Estrogens (Oral Contraceptives, HRT)**: May increase requirement for levothyroxine; monitor TSH. ## Monitoring & Follow-up - **Before Treatment**: Baseline TSH, Free T4 (fT4). - **During Treatment**: TSH and fT4 levels typically every **4-8 weeks** during initiation/titration. - **During Treatment**: Once stable, TSH levels every **6-12 months**. - **Clinical Signs**: Monitor for symptoms of hypo- or hyperthyroidism (fatigue, weight changes, palpitations, heat/cold intolerance). ## Clinical Pearls - 💡 **Timing**: Take **first thing in the morning**, at least **30-60 minutes before breakfast** or other medications, with water. - 💡 **Consistency**: Take at the **same time each day**. **Do not switch brands** without consulting a healthcare provider. - 💡 **Pediatric Adherence**: Crushing tablets for infants/children must be done immediately prior to administration. - 💡 **Pregnancy**: Levothyroxine requirements often **increase** during pregnancy (monitor TSH every trimester). > **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.