How do you adjust the dose of Synthroid (levothyroxine) in a patient with hypothyroidism, considering their thyroid function tests, including thyroid-stimulating hormone (TSH) and free T4 levels, and medical history?

Adjusting Synthroid (Levothyroxine) Dose

Adjust levothyroxine by 12.5-25 mcg increments every 6-8 weeks based on TSH levels, targeting a TSH of 0.5-4.5 mIU/L for most patients, with smaller increments (12.5 mcg) for elderly or cardiac patients. 1

Initial Assessment Before Dose Adjustment

Before making any dose changes, confirm the need for adjustment with repeat testing:

• Recheck TSH and free T4 after 6-8 weeks following any dose change, as this represents the time needed to reach steady state 1, 2
• Measure both TSH and free T4 to distinguish between adequate replacement, undertreatment, and overtreatment 1
• Never adjust doses more frequently than every 6-8 weeks—this is a critical pitfall that leads to inappropriate dosing before steady state is reached 1

Dose Adjustment Algorithm Based on TSH Results

When TSH is Elevated (>4.5 mIU/L)

For patients <70 years without cardiac disease:

• Increase levothyroxine by 25 mcg increments every 6-8 weeks until TSH normalizes 1
• Target TSH range: 0.5-4.5 mIU/L with normal free T4 1

For patients >70 years OR with cardiac disease:

• Use smaller increments of 12.5 mcg every 6-8 weeks to avoid cardiac complications 1
• Titrate more slowly and monitor closely for angina, palpitations, or arrhythmias 1

For severely elevated TSH (>10 mIU/L):

• Increase dose regardless of symptoms, as this carries ~5% annual risk of progression to overt hypothyroidism 1
• More aggressive titration may be appropriate in younger patients without cardiac disease 1

When TSH is Suppressed (<0.1 mIU/L)

Immediate dose reduction is mandatory to prevent serious complications:

• Decrease levothyroxine by 25-50 mcg for TSH <0.1 mIU/L 1
• Decrease by 12.5-25 mcg for TSH 0.1-0.45 mIU/L, particularly in elderly or cardiac patients 1

Critical risks of TSH suppression include:

• 3-5 fold increased risk of atrial fibrillation, especially in patients >60 years 1
• Increased risk of osteoporosis and fractures, particularly in postmenopausal women 1
• Potential increased cardiovascular mortality 1

When TSH is in Target Range (0.5-4.5 mIU/L)

• No dose adjustment needed if patient is clinically euthyroid 1
• Continue monitoring TSH annually or sooner if symptoms change 1
• Avoid the common pitfall of adjusting doses based on normal TSH values—approximately 25% of patients are unintentionally overtreated 1

Special Populations Requiring Modified Approach

Pregnant Patients

• Increase levothyroxine dose by 25-50% immediately upon pregnancy confirmation 1, 2
• Target TSH <2.5 mIU/L in first trimester, then trimester-specific ranges 1
• Check TSH every 4 weeks until stable, then at minimum once per trimester 1

Elderly Patients (>70 years)

• Start with lower doses (25-50 mcg/day) and use smaller increments (12.5 mcg) 1
• Target TSH may be slightly higher (up to 5-6 mIU/L may be acceptable in very elderly patients to avoid overtreatment risks) 1
• Monitor more carefully for cardiac complications and bone loss 1

Cardiac Patients

• Use conservative dose increases (12.5 mcg increments) every 6-8 weeks 1
• Consider repeating testing within 2 weeks rather than 6-8 weeks if patient has atrial fibrillation or serious cardiac conditions 1
• Obtain ECG to screen for arrhythmias before and during dose adjustments 1

Thyroid Cancer Patients

• Consult with endocrinologist before any dose adjustment, as TSH targets vary by risk stratification 1
• Low-risk patients: Target TSH 0.5-2 mIU/L 1
• Intermediate-to-high risk patients: Target TSH 0.1-0.5 mIU/L 1
• Structural incomplete response: May require TSH <0.1 mIU/L 1

Monitoring Schedule

During dose titration:

• Recheck TSH and free T4 every 6-8 weeks after each dose adjustment 1, 2
• Free T4 helps interpret ongoing abnormal TSH levels, as TSH may take longer to normalize 1

Once stable on maintenance dose:

• Monitor TSH every 6-12 months 1
• Recheck sooner if symptoms change or clinical status changes 1

For pregnant patients:

• Check TSH every 4 weeks until stable, then at minimum once per trimester 1

Critical Pitfalls to Avoid

• Never start thyroid hormone before ruling out adrenal insufficiency in suspected central hypothyroidism—this can precipitate life-threatening adrenal crisis 1
• Avoid excessive dose increases that could lead to iatrogenic hyperthyroidism, which occurs in 14-21% of treated patients 1
• Do not adjust doses too frequently—wait full 6-8 weeks between adjustments to reach steady state 1
• Never ignore suppressed TSH in elderly patients—this directly causes bone loss and cardiovascular complications 1
• Failing to distinguish between patients requiring TSH suppression (thyroid cancer) versus those who don't (primary hypothyroidism) leads to inappropriate management 1

Factors Affecting Levothyroxine Requirements

Be aware that dose adjustments may be needed with:

• Weight changes: Dose is approximately 1.6 mcg/kg/day for full replacement 1, 2
• Pregnancy: Requirements increase 25-50% 1
• Drug interactions: Iron, calcium, PPIs reduce absorption; enzyme inducers reduce efficacy 1, 3
• Malabsorption: Consider if TSH remains elevated despite apparently adequate doses 1, 4
• Food interactions: Soybean products, coffee, and certain foods may affect absorption 2

When to Consider Non-Compliance vs. True Dose Inadequacy

If TSH remains elevated despite dose increases:

• First assess compliance: Take levothyroxine on empty stomach, 30-60 minutes before food 1
• Check for drug interactions (iron, calcium, PPIs taken within 4 hours) 1
• Consider malabsorption if doses >300 mcg/day are required 2
• Rule out transient causes: acute illness, recovery from thyroiditis, recent iodine exposure 1