What is the recommended dosage of levothyroxine (T4) for a patient with hypothyroidism, considering factors such as age, weight, and history of heart disease?

Levothyroxine Dosing for Hypothyroidism

Initial Dosing Strategy

For adults under 70 years without cardiac disease, start levothyroxine at the full replacement dose of 1.6 mcg/kg/day, which safely achieves euthyroidism faster than low-dose titration without increasing cardiac risk. 1, 2, 3

Age and Cardiac Risk-Based Dosing

Patients under 70 years without cardiac disease:

• Start at 1.6 mcg/kg/day (full replacement dose) 1, 2, 4
• Titrate by 12.5-25 mcg every 4-6 weeks until TSH normalizes to 0.5-4.5 mIU/L 1, 2
• A prospective randomized trial demonstrated this approach is safe and reaches euthyroidism significantly faster than low-dose titration (13 vs 1 patient euthyroid at 4 weeks, p=0.005), with no cardiac events documented 3

Patients over 70 years OR with cardiac disease:

• Start at 25-50 mcg/day (lower dose) 1, 2, 4
• Titrate by 12.5-25 mcg every 6-8 weeks 1, 2
• This conservative approach prevents unmasking cardiac ischemia or precipitating arrhythmias 1, 5
• Patients with known or suspected ischemic heart disease require 12.5-50 mcg/day starting dose 4

Weight-Based Calculations

The full replacement dose calculation is straightforward: multiply patient weight in kg by 1.6 mcg 1, 2, 6. For example, a 70 kg patient without cardiac disease would start at approximately 112 mcg daily (typically rounded to 100-125 mcg available tablet strengths) 2.

Critical Pre-Treatment Considerations

Before initiating levothyroxine, always rule out concurrent adrenal insufficiency, as starting thyroid hormone before corticosteroids can precipitate life-threatening adrenal crisis. 1

• In suspected central hypothyroidism or hypophysitis, start physiologic dose steroids 1 week prior to thyroid hormone replacement 1
• This is particularly important in patients with autoimmune hypothyroidism who have increased risk of concurrent autoimmune adrenal insufficiency 1

Monitoring and Titration

Recheck TSH and free T4 every 6-8 weeks after any dose adjustment, as levothyroxine requires this interval to reach steady state. 1, 2, 7

• Target TSH: 0.5-4.5 mIU/L with normal free T4 1, 8
• Once stable, monitor TSH annually or when symptoms change 1, 7
• The peak therapeutic effect may not be attained for 4-6 weeks 2

Dose Adjustment Increments

• Use 12.5-25 mcg increments based on current dose and patient characteristics 1, 2
• Smaller increments (12.5 mcg) for elderly or cardiac patients 1
• Larger increments (25 mcg) acceptable for younger patients without cardiac disease 1

Special Population: Pregnancy

Women with pre-existing hypothyroidism should increase their levothyroxine dose by 30% (approximately 2 extra doses per week) immediately upon pregnancy confirmation. 4

• Target TSH <2.5 mIU/L in first trimester 1
• Monitor TSH every 4 weeks during first half of pregnancy, then at minimum each trimester 1, 2
• Levothyroxine requirements increase by 25-50% during pregnancy 1
• Untreated hypothyroidism increases risk of preeclampsia, low birth weight, and neurodevelopmental effects 1, 4

Common Pitfalls to Avoid

Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, fractures, and cardiac complications. 1

• Never adjust doses more frequently than every 6-8 weeks before steady state is reached 1
• Avoid starting at full replacement dose in elderly or cardiac patients—this can precipitate myocardial infarction or fatal arrhythmias 1
• Do not treat based on single elevated TSH value, as 30-60% normalize spontaneously on repeat testing 1, 5
• TSH suppression (<0.1 mIU/L) increases atrial fibrillation risk 3-5 fold and fracture risk, especially in patients over 60 1

Administration Guidelines

• Take on empty stomach, 30-60 minutes before breakfast 1, 2
• Separate from iron, calcium supplements, or antacids by at least 4 hours 1, 2
• Avoid soybean-based infant formula which decreases absorption 2

Treatment Targets by Indication

Primary hypothyroidism: TSH 0.5-4.5 mIU/L with normal free T4 1, 8, 2

Secondary/tertiary hypothyroidism: TSH unreliable; target free T4 in upper half of normal range 2

Thyroid cancer (risk-stratified):

• Low-risk: TSH 0.5-2 mIU/L 1
• Intermediate-risk: TSH 0.1-0.5 mIU/L 1
• High-risk/structural incomplete response: TSH <0.1 mIU/L 1