What is the recommended levothyroxine (thyroxine) tablet dose based on body weight for adults, children, and special populations such as the elderly, patients with cardiovascular disease, and pregnant patients?

Levothyroxine Dosing Based on Body Weight

Standard Weight-Based Dosing for Adults

For adults under 70 years without cardiac disease, start levothyroxine at approximately 1.6 mcg/kg/day of actual body weight as the full replacement dose. 1, 2

• This represents the standard FDA-approved starting dose for most adult patients with primary hypothyroidism 2
• Titrate by 12.5-25 mcg increments every 4-6 weeks based on TSH levels until the patient achieves euthyroidism (TSH 0.5-4.5 mIU/L) 1, 2
• Monitor TSH every 6-8 weeks during dose titration, then every 6-12 months once stable 1

Modified Dosing for Elderly Patients (≥70 Years)

For patients over 70 years old, start with a significantly lower dose of 25-50 mcg/day regardless of body weight, then titrate slowly every 6-8 weeks. 1, 2

• Elderly patients require approximately one-third less levothyroxine than younger adults—the mean euthyroid dose is 1.09 mcg/kg actual body weight (or 1.35 mcg/kg ideal body weight) 3
• 84% of euthyroid elderly individuals achieve target TSH on doses less than 1.6 mcg/kg 3
• Slower titration prevents cardiac complications including angina, arrhythmias, and myocardial infarction 1, 2

Dosing for Patients with Cardiac Disease

For any patient with underlying cardiac disease or at risk for atrial fibrillation, start at 25-50 mcg/day and titrate by 12.5-25 mcg increments every 6-8 weeks. 1, 2

• This conservative approach applies regardless of age to avoid unmasking cardiac ischemia or precipitating heart failure 1
• Even therapeutic doses of levothyroxine can worsen angina or trigger arrhythmias in patients with coronary disease 1

Dosing for Obese Patients

For obese patients, calculate the starting dose using ideal body weight rather than actual body weight to avoid overtreatment. 4

• The recommended dose is approximately 2.3 mcg/kg of lean body mass, which remains consistent across all BMI ranges 4
• Using actual body weight in obese patients results in excessive dosing—obese individuals require only 0.9 mcg/kg actual body weight versus 1.14 mcg/kg in non-obese patients 3
• Start conservatively at 100-125 mcg daily for obese patients and adjust based on TSH response after 6-8 weeks 1

Pediatric Dosing by Age

Levothyroxine requirements in children are substantially higher per kilogram than adults and decrease progressively with 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 and puberty complete: 1.6 mcg/kg/day 2

Adjust dosage every 2 weeks in pediatric patients based on TSH or free T4 until euthyroid 2

Pregnancy Dosing

For pregnant patients with pre-existing hypothyroidism, increase the levothyroxine dose by 25-50% immediately upon pregnancy confirmation. 1, 2

• Levothyroxine requirements increase by 25-50% during pregnancy in most women with pre-existing hypothyroidism 1
• Target TSH <2.5 mIU/L in the first trimester 1
• Monitor TSH and free T4 every 4 weeks until stable, then at minimum once per trimester 1
• Increase dose by 12.5-25 mcg increments based on TSH results 1, 2

Post-Thyroidectomy Dosing

After total thyroidectomy for benign disease, the optimal starting dose varies by age and BMI, ranging from 1.4-1.8 mcg/kg/day rather than a fixed 1.6 mcg/kg/day. 5

• Younger patients with lower BMI require doses closer to 1.8 mcg/kg/day 5
• Older patients with higher BMI require doses closer to 1.4 mcg/kg/day 5
• Using an age- and BMI-adjusted nomogram achieves euthyroidism in 68% of patients at first follow-up versus only 41% with standard weight-based dosing 5
• For soft-gel levothyroxine formulations specifically, athyreotic patients may require lower doses—the most accurate cut-off to achieve TSH <1.0 mIU/L is 1.53 mcg/kg/day 6

Critical Safety Considerations Before Initiating Therapy

Before starting levothyroxine in any patient, rule out concurrent adrenal insufficiency by checking morning cortisol and ACTH, especially in suspected central hypothyroidism. 1

• Starting thyroid hormone before adequate corticosteroid coverage can precipitate life-threatening adrenal crisis 1
• If adrenal insufficiency is confirmed, start hydrocortisone (20 mg morning, 10 mg afternoon) for at least one week before initiating levothyroxine 1

Common Pitfalls to Avoid

• Never use actual body weight for obese patients—this leads to overtreatment in 25% of cases, increasing risks of atrial fibrillation, osteoporosis, and fractures 1, 4
• Never start elderly patients at full replacement doses—this can precipitate cardiac decompensation even in previously stable patients 1, 3
• Never adjust doses more frequently than every 6-8 weeks—levothyroxine requires 4-6 weeks to reach steady state 1, 2
• Never ignore age-related changes—thyroid hormone metabolism slows with advancing age, requiring dose reductions of approximately 30% in elderly populations 3