First Follow-Up Lab After Starting Levothyroxine
Recheck TSH and free T4 at 6–8 weeks after initiating levothyroxine or after any dose adjustment. This interval is required for levothyroxine to reach steady-state serum concentrations and for TSH to equilibrate in response to the new thyroid hormone level 1, 2, 3, 4.
Why 6–8 Weeks Is the Standard Interval
- Levothyroxine has a long half-life (approximately 7 days), meaning it takes roughly 5–6 weeks to achieve stable blood levels after starting or changing the dose 5.
- TSH normalization lags behind T4 normalization. Even when serum T4 and T3 reach therapeutic levels within 2–3 weeks, TSH may take an additional 3–4 weeks to fully suppress or normalize 6. In newly treated hypothyroid patients, TSH may not fall below 20 mIU/L until day 21–23 and may not reach the normal range until 5–6 weeks 6.
- Free T4 can help interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize even when peripheral thyroid hormone levels are adequate 1, 2.
Monitoring Protocol by Clinical Context
Standard Adult Primary Hypothyroidism
- First follow-up: TSH and free T4 at 6–8 weeks after starting levothyroxine 1, 3, 4.
- After dose adjustments: Repeat TSH and free T4 every 6–8 weeks until TSH is within the target range (0.5–4.5 mIU/L) 1, 2.
- Once stable: Check TSH every 6–12 months or sooner if symptoms change 1, 3, 4.
Patients with Cardiac Disease or Atrial Fibrillation
- More frequent monitoring may be warranted—consider repeating testing within 2 weeks of dose adjustment rather than waiting the full 6–8 weeks, especially in elderly patients or those with serious cardiac conditions 1, 2.
- This accelerated schedule helps detect iatrogenic hyperthyroidism early, which can precipitate arrhythmias or worsen heart failure 1.
Pregnant Patients with Pre-Existing Hypothyroidism
- Check TSH and free T4 every 4 weeks after initiating or adjusting levothyroxine until a stable dose is achieved and TSH is within the trimester-specific reference range 3.
- Once stable, monitor at minimum once per trimester 3.
- Levothyroxine requirements typically increase by 25–50% during pregnancy, necessitating closer surveillance 1.
Pediatric Patients
- Monitor TSH and total or free T4 at 2 and 4 weeks after initiating treatment 3.
- Recheck 2 weeks after any dose change 3.
- Once dosage is stabilized, monitor every 3–12 months until growth is completed 3.
- Failure of serum T4 to increase into the upper half of the normal range within 2 weeks, or TSH to decrease below 20 IU/L within 4 weeks, may indicate inadequate therapy 3.
Patients on Immune Checkpoint Inhibitors
- Check TSH (with optional free T4) every 4–6 weeks as part of routine monitoring for asymptomatic patients 1.
- Thyroid dysfunction occurs in 6–9% with anti-PD-1/PD-L1 therapy and 16–20% with combination immunotherapy 1.
Common Pitfalls to Avoid
- Do not recheck labs before 6 weeks unless there is a clinical emergency (e.g., cardiac symptoms, suspected myxedema coma). Testing too early, before steady state is reached, leads to inappropriate dose adjustments 1, 7.
- Do not rely on TSH alone during the titration phase. Free T4 provides critical information when TSH remains abnormal, as TSH normalization lags behind peripheral hormone levels 1, 2, 6.
- Avoid excessive dose increases that could lead to iatrogenic hyperthyroidism. Approximately 25% of patients on levothyroxine are unintentionally overtreated, increasing risks for atrial fibrillation, osteoporosis, and cardiovascular complications 1, 4.
- Never start thyroid hormone before ruling out adrenal insufficiency in patients with suspected central hypothyroidism, as this can precipitate adrenal crisis 1, 2.
Target TSH Range
- For primary hypothyroidism: Target TSH 0.5–4.5 mIU/L with normal free T4 1, 3, 4.
- For thyroid cancer patients: TSH targets vary by risk stratification and require endocrinologist guidance 1.
- For pregnant patients: Maintain TSH in the trimester-specific reference range, ideally <2.5 mIU/L in the first trimester 1, 3.