When to Check Thyroid Function Before Adjusting Levothyroxine
Recheck TSH and free T4 every 6–8 weeks after any levothyroxine dose adjustment until the target TSH range (0.5–4.5 mIU/L) is achieved, then monitor every 6–12 months once stable. 1
Standard Monitoring Timeline During Dose Titration
The 6–8 week interval is critical because levothyroxine requires this time to reach steady-state concentrations in the body. 1 Testing earlier than 6 weeks will capture transitional values that do not reflect the true effect of your dose change, potentially leading to inappropriate adjustments. 1
- After initiating levothyroxine or changing the dose: Measure TSH and free T4 at 6–8 weeks to evaluate response 1, 2
- Continue 6–8 week monitoring intervals until TSH normalizes within the reference range (0.5–4.5 mIU/L) 1
- Free T4 helps interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize than free T4 1
Long-Term Monitoring After Stabilization
Once you achieve a stable dose with TSH in the target range, switch to monitoring every 6–12 months. 1 This less frequent schedule is appropriate because thyroid function typically remains stable on a consistent levothyroxine dose.
- Recheck TSH annually (or every 6 months if preferred) once adequately treated 1
- Test sooner if symptoms change or new medications are started 1
- About 25% of patients are unintentionally maintained on doses that suppress TSH, increasing risks for atrial fibrillation, osteoporosis, and cardiac complications 1
Dose Adjustment Strategy
When TSH remains elevated, increase levothyroxine by 12.5–25 mcg based on the patient's age and cardiac status. 1, 2
- For patients <70 years without cardiac disease: Use 25 mcg increments for more aggressive titration 1
- For patients >70 years or with cardiac disease: Use smaller 12.5 mcg increments to avoid cardiac complications 1, 2
- Larger adjustments risk iatrogenic hyperthyroidism and should be avoided 1
Special Populations Requiring Modified Timing
Cardiac Disease or Elderly Patients
For patients with atrial fibrillation, cardiac disease, or serious medical conditions, consider repeating testing within 2 weeks rather than waiting the full 6–8 weeks. 1 This accelerated monitoring helps detect overtreatment that could precipitate cardiac complications.
Pregnant Women
Pregnant women require TSH monitoring every 4–6 weeks (not 6–8 weeks) after dose adjustments, as levothyroxine requirements typically increase by 25–50% during pregnancy. 1, 2 Once stabilized, check TSH at minimum once per trimester. 1
Patients on Immune Checkpoint Inhibitors
Monitor TSH every 4–6 weeks for the first 3 months of immunotherapy, then every second cycle thereafter. 1 Thyroid dysfunction occurs in 6–9% with anti-PD-1/PD-L1 therapy and 16% with combination immunotherapy. 1
Common Pitfalls to Avoid
- Never adjust doses more frequently than every 6–8 weeks (except in urgent cardiac situations), as this leads to chasing transitional values before steady state is reached 1
- Do not skip free T4 measurement when TSH remains abnormal during titration—it helps distinguish between inadequate dosing and delayed TSH normalization 1
- Development of low TSH (<0.1–0.45 mIU/L) suggests overtreatment; reduce dose by 12.5–25 mcg and recheck in 6–8 weeks 1