What are the available doses of levothyroxine (T4) for treating hypothyroidism in patients of different ages and weights?

Levothyroxine Available Doses for Prescription

Levothyroxine tablets are available in a wide range of strengths from 25 mcg to 300 mcg, with most patients requiring 100-150 mcg daily for primary hypothyroidism. 1, 2

Standard Available Tablet Strengths

The FDA-approved levothyroxine formulations include the following tablet strengths 2:

• 25 mcg - Used for initial dosing in elderly patients, those with cardiac disease, or for small dose adjustments 1, 2
• 50 mcg - Common starting dose for at-risk populations 1, 2
• 75 mcg - Intermediate strength for dose titration 2, 3
• 88 mcg - Intermediate strength often needed for precise dosing 3
• 100 mcg - One of the most commonly prescribed maintenance doses 1, 4
• 112 mcg - Intermediate strength for fine-tuning 3
• 125 mcg - Median maintenance dose for most patients 1, 4
• 137 mcg - Intermediate strength 3
• 150 mcg - Upper range of common maintenance doses 1, 4
• 175 mcg - Higher maintenance dose 2
• 200 mcg - Higher maintenance dose 2
• 300 mcg - Maximum commonly used dose 2

Dosing by Patient Population

Adults with Primary Hypothyroidism (Age <70, No Cardiac Disease)

• Full replacement dose: 1.6 mcg/kg/day 1, 2, 5
• Most patients (65%) require 100-150 mcg daily 1, 4
• Median maintenance dose is 125 mcg daily 1, 4
• Titrate by 12.5-25 mcg increments every 4-6 weeks until TSH normalizes 1, 2

Elderly Patients (Age >70) or Those with Cardiac Disease

• Starting dose: 25-50 mcg daily 1, 2, 5, 6
• Elderly patients typically require one-third less than younger adults 6
• Mean physiologic replacement in elderly: 110-113 mcg daily 6
• Titrate every 6-8 weeks with smaller increments (12.5 mcg) to avoid cardiac complications 1, 2

Pediatric Patients (Weight-Based Dosing)

The starting daily dosage varies significantly by age 2:

• 0-3 months: 10-15 mcg/kg/day 2
• 3-6 months: 8-10 mcg/kg/day 2
• 6-12 months: 6-8 mcg/kg/day 2
• 1-5 years: 5-6 mcg/kg/day 2
• 6-12 years: 4-5 mcg/kg/day 2
• >12 years (growth incomplete): 2-3 mcg/kg/day 2
• Growth complete: 1.6 mcg/kg/day 2

Pregnant Patients

• Pre-existing hypothyroidism: Increase pre-pregnancy dose by 25-50% immediately upon pregnancy confirmation 1, 2, 5
• New-onset hypothyroidism with TSH ≥10: 1.6 mcg/kg/day 2
• New-onset hypothyroidism with TSH <10: 1.0 mcg/kg/day 2
• Adjust by 12.5-25 mcg increments every 4 weeks to maintain TSH in trimester-specific range 1, 2

Post-Thyroidectomy Patients

• After total thyroidectomy: 1.5 mcg/kg/day based on actual body weight 7
• After lobectomy: 1.3 mcg/kg/day based on actual body weight 7
• This weight-based calculation is currently the most reliable method for post-surgical dosing 7

Clinical Considerations for Dose Selection

Maximum Dosing Thresholds

• Dosages >200 mcg/day are seldom required 2
• Inadequate response to >300 mcg/day is rare and suggests poor compliance, malabsorption, or drug interactions rather than need for higher doses 2

Importance of Intermediate Strengths

• Many formulations have large intervals between tablet strengths at the lower end (25,50,75 mcg), which can be a barrier to optimal dosing 3
• Intermediate tablet strengths (88 mcg, 112 mcg, 137 mcg) facilitate precise dose titration and enable convenient single-tablet daily regimens 3
• Nearly half of patients on levothyroxine demonstrate either under- or over-treatment, highlighting the need for precise dosing options 3

Predicting Optimal Dose

• Pretreatment TSH levels correlate with optimal levothyroxine dose in a curvilinear relationship 4
• Higher pretreatment TSH generally predicts need for higher replacement doses 4
• However, individual patient factors (age, weight, cardiac status) must override any formula-based predictions 1, 2

Common Pitfalls

• Approximately 25% of patients are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, and cardiac complications 1
• Starting elderly or cardiac patients at full replacement doses can precipitate myocardial infarction, heart failure, or arrhythmias 1, 5
• Adjusting doses too frequently before reaching steady state (should wait 4-6 weeks between adjustments) leads to inappropriate dosing 1, 2