What is the recommended dose of levothyroxine (T4) for hypothyroidism?

Levothyroxine Dosing for Hypothyroidism

For adults under 70 years without cardiac disease, start levothyroxine at the full replacement dose of 1.6 mcg/kg/day (typically 75-100 mcg for women, 100-150 mcg for men), which safely achieves euthyroidism faster than low-dose titration without increasing cardiac risk. 1, 2, 3, 4

Initial Dosing Strategy

For younger, healthy adults (age <70, no cardiac disease):

• Start with full replacement dose of 1.6 mcg/kg/day 1, 3, 5
• This typically translates to 75-100 mcg/day for women and 100-150 mcg/day for men 5
• A prospective randomized trial demonstrated this approach is safe and achieves euthyroidism significantly faster than low-dose titration (13 vs 1 patient euthyroid at 4 weeks, p=0.005) with no cardiac events 4

For elderly patients (>70 years) or those with cardiac disease:

• Start with 25-50 mcg/day 1, 2, 3
• Use smaller increments (12.5 mcg) during titration to minimize cardiovascular risk 1, 2
• Older patients require less thyroid hormone—doses of 100 mcg/day or less are common over age 40, and some patients over 60 need only 50 mcg/day 6

Dose Titration Protocol

Adjustment intervals and increments:

• Recheck TSH and free T4 every 6-8 weeks after any dose change 1, 2, 3
• Adjust dose in 12.5-25 mcg increments based on TSH response 1, 2, 3
• For elderly or cardiac patients, use smaller 12.5 mcg increments 1, 2
• Peak therapeutic effect may not occur for 4-6 weeks after dose adjustment 3

Target TSH levels:

• For primary hypothyroidism: TSH 0.5-4.5 mIU/L with normal free T4 1
• For secondary/tertiary hypothyroidism: Use free T4 (target upper half of normal range) rather than TSH for monitoring 3

Critical Safety Considerations

Before initiating levothyroxine:

• Always rule out adrenal insufficiency first—starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis 1, 2
• In suspected central hypothyroidism or hypophysitis, start physiologic dose steroids at least 1 week before thyroid hormone 1, 2

Monitoring after stabilization:

• Once TSH normalizes on stable dose, recheck every 6-12 months 1
• More frequent monitoring (every 2 weeks) warranted for patients with atrial fibrillation or serious cardiac conditions 1

Common Pitfalls to Avoid

Overtreatment risks (TSH <0.1 mIU/L):

• Occurs in 14-21% of treated patients and approximately 25% are unintentionally maintained on excessive doses 1, 2
• Increases risk for atrial fibrillation (5-fold in patients ≥45 years), osteoporosis, fractures, and cardiac complications 1, 2
• Requires immediate dose reduction by 25-50 mcg 1

Undertreatment risks (TSH persistently >10 mIU/L):

• Indicates insufficient replacement requiring dose escalation 1, 2
• Associated with persistent symptoms, cardiovascular dysfunction, and adverse lipid profiles 1, 2

Dosing errors to avoid:

• Adjusting doses too frequently before reaching steady state—wait full 6-8 weeks between adjustments 1
• Treating based on single elevated TSH without confirmation—30-60% normalize spontaneously 1
• Failing to account for decreased requirements in elderly patients 6

Special Populations

Pregnant patients:

• Levothyroxine requirements increase 25-50% during pregnancy 1
• Check TSH as soon as pregnancy confirmed and each trimester 3
• Maintain TSH in trimester-specific reference range 3

Patients on immunotherapy:

• Thyroid dysfunction occurs in 6-9% with anti-PD-1/PD-L1 therapy 1
• Consider treatment even for subclinical hypothyroidism if fatigue present 1
• Continue immunotherapy in most cases—rarely requires interruption 1

Thyroid cancer patients requiring TSH suppression:

• Target TSH varies by risk: 0.5-2 mIU/L (low-risk), 0.1-0.5 mIU/L (intermediate-risk), <0.1 mIU/L (structural incomplete response) 1
• Requires endocrinologist consultation for target determination 1

Administration Considerations

• Administer at least 4 hours before or after drugs that interfere with absorption 3
• Evaluate need for dose adjustment when regularly administering within 1 hour of foods affecting absorption 3
• For pediatric patients unable to swallow tablets, crush and suspend in 5-10 mL water, administer immediately 3