Monitoring TSH and Free T4 in Patients on Levothyroxine
Monitor TSH and free T4 every 6-8 weeks during dose titration, then every 6-12 months once stable thyroid function is achieved. 1, 2, 3
During Initial Titration Phase
Check TSH and free T4 at 6-8 week intervals after starting levothyroxine or after any dose adjustment. 1, 2, 3 This timing is critical because:
- Levothyroxine has a long half-life, and the peak therapeutic effect of a given dose may not be attained for 4-6 weeks 3
- TSH may take longer to normalize than free T4, making both measurements valuable during titration 1, 2
- Adjusting doses more frequently than every 6-8 weeks before reaching steady state is a common pitfall to avoid 1
Adjust levothyroxine by 12.5-25 mcg increments every 6-8 weeks until TSH normalizes to the reference range (0.5-4.5 mIU/L). 1, 2, 3
After Achieving Stable Thyroid Function
Once the appropriate maintenance dose is established and TSH is within the reference range, monitor TSH every 6-12 months. 1, 2, 3, 4
- Annual monitoring is recommended for most stable patients 1, 4
- Some guidelines support checking as frequently as every 6 months during the first year after stabilization 2
- The FDA label specifies evaluation of clinical and biochemical response every 6-12 months in stable patients 3
Special Populations Requiring More Frequent Monitoring
Pregnant patients require TSH and free T4 monitoring every 4 weeks until stable, then at minimum during each trimester. 3
- For pre-existing hypothyroidism with TSH above the trimester-specific range, check every 4 weeks until stable 3
- Monitor TSH 4-8 weeks postpartum after reducing to pre-pregnancy dose 3
Patients with cardiac disease, atrial fibrillation, or serious medical conditions may need testing within 2 weeks of dose adjustment rather than waiting 6-8 weeks. 1
Elderly patients (>70 years) or those with cardiac disease require more careful monitoring after initiating lower starting doses (25-50 mcg/day). 1
When to Check Sooner Than Scheduled
Recheck TSH and free T4 sooner if:
- Symptoms of hypothyroidism or hyperthyroidism develop 1, 3
- New medications are started that may interact with levothyroxine absorption (iron, calcium, proton pump inhibitors) 1
- TSH becomes suppressed (<0.1 mIU/L), indicating potential overtreatment requiring dose reduction 1
- TSH remains >10 mIU/L despite treatment, suggesting inadequate dosing or compliance issues 1
Common Monitoring Pitfalls
Approximately 25% of patients on levothyroxine are unintentionally maintained on doses sufficient to fully suppress TSH, increasing risks for osteoporosis, fractures, and atrial fibrillation. 1 This highlights the critical importance of adhering to the recommended 6-12 month monitoring schedule for stable patients.
Failing to wait the full 6-8 weeks between dose adjustments leads to inappropriate titration and potential over- or under-treatment. 1, 2
Free T4 measurement helps interpret ongoing abnormal TSH levels during therapy, as TSH may lag behind free T4 changes. 1, 2 Both should be measured during titration, not TSH alone.