Synthroid Starting Dose for Adults
Standard Starting Dose Based on Age and Cardiac Status
For healthy adults under 60 years without cardiac disease, start levothyroxine at the full replacement dose of 1.6 mcg/kg/day. 1, 2, 3, 4
This approach is safe, reaches euthyroidism faster (13/25 patients at 4 weeks vs 1/25 with low-dose), and causes no cardiac events even in previously undiagnosed patients. 3 The full replacement dose is more convenient and cost-effective than gradual titration. 3
For adults over 60 years OR those with any cardiac disease (known or suspected ischemic heart disease, heart failure, arrhythmias), start at 25-50 mcg/day and titrate slowly. 1, 2, 5, 4, 6, 7
Age-Specific Dosing Rationale
Elderly patients require significantly lower levothyroxine doses due to decreased thyroxine degradation rates with aging. 6, 7
- Patients aged 75+ years average only 118 mcg/day for full replacement 6
- Patients aged 48 years average 158 mcg/day 6
- Some patients over 60 years need only 50 mcg/day or less 7
Starting elderly patients at full replacement doses risks unmasking cardiac ischemia, precipitating arrhythmias, or triggering heart failure. 1, 5, 4
Cardiac Risk Stratification Algorithm
High Cardiac Risk (Start 25-50 mcg/day):
- Age >60 years 2, 4, 6, 7
- Known coronary artery disease 5, 4
- History of myocardial infarction 4
- Heart failure 1
- Atrial fibrillation or arrhythmia history 1, 2
- Uncontrolled hypertension 1
Low Cardiac Risk (Start 1.6 mcg/kg/day):
- Age <60 years 2, 3, 4
- No cardiac symptoms 3
- Normal cardiac examination 3
- No cardiac disease history 2, 3
Titration Strategy
For low-risk patients on full replacement: Check TSH at 6-8 weeks, adjust by 12.5-25 mcg increments every 4-6 weeks until TSH normalizes (0.5-4.5 mIU/L). 1, 2
For high-risk patients on low starting dose: Increase by 12.5-25 mcg every 6-8 weeks based on TSH response and cardiac tolerance. 1, 2, 5 Smaller increments (12.5 mcg) are safer in elderly or cardiac patients. 1
Critical Safety Considerations
Before starting levothyroxine in ANY patient, rule out adrenal insufficiency by checking morning cortisol and ACTH, especially if central hypothyroidism is suspected. 1 Starting thyroid hormone before adequate glucocorticoid coverage can precipitate life-threatening adrenal crisis. 1
Monitor elderly and cardiac patients closely during titration for angina, palpitations, dyspnea, or worsening heart failure. 1, 5, 4 Even therapeutic levothyroxine doses can unmask underlying coronary disease in elderly patients. 1
Common Pitfalls
Avoid starting elderly patients at full replacement doses (1.6 mcg/kg/day) even if they appear healthy—age alone mandates lower starting doses due to decreased thyroxine clearance. 6, 7
Avoid excessive dose increases that could lead to iatrogenic hyperthyroidism, which increases risk for atrial fibrillation (3-5 fold), osteoporosis, fractures, and cardiovascular mortality, especially in patients over 60 years. 1
Approximately 25% of patients on levothyroxine are unintentionally overtreated with suppressed TSH, increasing serious complication risks. 1