How often should TSH (Thyroid-Stimulating Hormone) levels be checked after starting levothyroxine (T4) therapy in a patient newly diagnosed with hypothyroidism?

TSH Monitoring Frequency After Starting Levothyroxine

Monitor TSH every 6-8 weeks after starting levothyroxine or after any dose adjustment until the target TSH range is achieved, then check annually once stable. 1, 2, 3

Initial Titration Phase (First Few Months)

During dose titration:

• Check TSH and free T4 every 6-8 weeks after initiating therapy or changing the dose 1, 2, 3
• Wait the full 6-8 weeks between dose adjustments to allow steady-state levels to be reached, as levothyroxine has a long half-life 1, 3
• TSH may lag behind actual thyroid status by several weeks, which is why free T4 measurement helps interpret ongoing abnormal TSH levels during therapy 1, 2
• Continue this 6-8 week monitoring interval until TSH normalizes to the reference range (0.5-4.5 mIU/L) 1

Common pitfall to avoid: Adjusting doses too frequently before reaching steady state leads to overcorrection or undercorrection 1. The 6-8 week interval is critical because normalization of serum TSH may take several weeks even after T4 and T3 normalize 4.

Maintenance Phase (After Stabilization)

Once adequately treated on a stable dose:

• Check TSH every 6-12 months 1, 3, 5
• Recheck sooner if symptoms change or clinical status changes 1, 3
• Annual testing is sufficient to ensure appropriate replacement and avoid the 25% of patients who are unintentionally maintained on doses sufficient to fully suppress TSH 1, 2

Special Populations Requiring Modified Monitoring

Patients with cardiac disease, atrial fibrillation, or serious medical conditions:

• Consider repeating testing within 2 weeks rather than waiting the full 6-8 weeks after dose adjustments 1, 2
• More frequent monitoring warranted due to higher risk of cardiac complications from overtreatment 1, 2

Elderly patients (>70 years) or those with coronary artery disease:

• Start with lower doses (25-50 mcg/day) and monitor at the same 6-8 week intervals 1, 5, 6
• These patients are at increased risk of cardiac decompensation even with therapeutic doses 1

Pregnant patients with pre-existing hypothyroidism:

• Monitor TSH every 4 weeks during pregnancy until stable, then at minimum during each trimester 3
• Levothyroxine requirements typically increase 25-50% during pregnancy 1
• Reduce to pre-pregnancy dose immediately after delivery and recheck TSH 4-8 weeks postpartum 3

Critical Monitoring Considerations

Why the 6-8 week interval matters:

• Studies show that while T4 and T3 normalize within 3 weeks of starting levothyroxine, TSH normalization may take several additional weeks 4
• In untreated hypothyroid patients started on full replacement, serum TSH did not fall consistently below 20 mIU/L until day 21-23 and did not reach normal range until day 37-42 4

Risks of inadequate monitoring:

• Approximately 25% of patients on levothyroxine are unintentionally maintained on excessive doses that fully suppress TSH, increasing risks for atrial fibrillation, osteoporosis, fractures, and cardiac complications 1, 2
• Both overtreatment and undertreatment are associated with cardiovascular health risks 5

What to measure:

• TSH is the primary monitoring test with sensitivity above 98% and specificity greater than 92% 1
• Free T4 should also be measured during titration to help interpret ongoing abnormal TSH levels, as TSH may take longer to normalize 1, 2
• Once stable, TSH alone is typically sufficient for annual monitoring 1