What labs should be ordered to monitor hypothyroidism?

Laboratory Tests for Monitoring Hypothyroidism

Thyroid stimulating hormone (TSH) is the primary laboratory test for monitoring hypothyroidism, with free thyroxine (free T4) used as an adjunctive test when necessary. 1

Primary Monitoring Test

• TSH: This is the most sensitive test for monitoring adequacy of thyroid hormone replacement in primary hypothyroidism.
  • Normal range: 0.3-5.7 μIU/mL (may vary by laboratory)
  • Goal: Keep TSH within normal range for most patients
  • Frequency: Every 4-6 weeks after any dosage change, then every 6-12 months once stable 1

Secondary Monitoring Tests

• Free T4: Used in conjunction with TSH in certain situations:

  • When TSH results are inconsistent with clinical presentation
  • When central (secondary) hypothyroidism is suspected
  • During initial treatment phase
  • When monitoring patients with thyroid cancer on suppressive therapy 1, 2

• Free T3: Rarely needed for routine monitoring but may be useful in:

  • Cases where TSH is undetectable but free T4 is normal
  • Patients with persistent symptoms despite normal TSH and free T4 3

Monitoring Algorithm

1. Initial diagnosis and treatment phase:

   • Check TSH and free T4 every 4-6 weeks until stable dose achieved 1
   • Note: After starting levothyroxine, free T4 normalizes within 1-2 weeks, but TSH may take 6-8 weeks to normalize 4

2. Stable patients:

   • Monitor TSH every 6-12 months 1
   • If symptoms change or medication adjustments occur, recheck TSH after 6-8 weeks

3. Special situations:

   • Pregnancy: Check TSH and free T4 as soon as pregnancy is confirmed and at minimum during each trimester 5
   • Central hypothyroidism: Monitor free T4 levels (not TSH) and maintain in upper half of normal range 5, 3
   • Thyroid cancer patients on suppressive therapy: Monitor both TSH and free T4 2

Clinical Pearls and Pitfalls

• Timing matters: TSH levels show diurnal variation; morning samples (around 8 AM) are preferred for consistency 1

• Medication interference: TSH should be measured before the daily dose of levothyroxine or at least 4 hours after taking it

• Overtreatment indicators: Low TSH with normal or elevated free T4 suggests excessive thyroid hormone replacement 1

• Undertreatment indicators: Elevated TSH with normal or low free T4 suggests inadequate replacement 1

• Common pitfall: Relying solely on symptom improvement without laboratory confirmation can lead to under or overtreatment

• Important caveat: After initiating treatment, T4 levels normalize more quickly than TSH levels, which may take 3-6 weeks to reach steady state 4

By following this monitoring approach, clinicians can effectively manage hypothyroidism, minimize complications related to under or overtreatment, and optimize patient outcomes in terms of morbidity, mortality, and quality of life.