What labs are used to diagnose and monitor hypothyroidism and how often should they be checked?

Laboratory Testing and Monitoring for Hypothyroidism

Serum TSH is the primary screening test for hypothyroidism, with free T4 used for confirmation, and monitoring should occur every 6-12 months once stable. 1

Diagnostic Laboratory Testing

Initial Screening

• Serum TSH is the most reliable primary screening test with high sensitivity (98%) and specificity (92%) 1
• If TSH is elevated, measure free T4 (FT4) to differentiate between:
  • Subclinical hypothyroidism: Elevated TSH with normal free T4
  • Overt hypothyroidism: Elevated TSH with low free T4 1

Special Testing Situations

• For suspected central (secondary/tertiary) hypothyroidism:
  • Measure both TSH and free T4 simultaneously 1
  • TSH may be inappropriately normal or only slightly elevated in central hypothyroidism 2

Monitoring Frequency

Primary Hypothyroidism

• Initial dose adjustment period:
  • Check TSH 6-8 weeks after starting treatment or changing dosage 3
  • Once stable, monitor TSH every 6-12 months 1

Special Populations

• Pediatric patients:

  • Monitor TSH and total or free T4 at 2 and 4 weeks after treatment initiation
  • Check 2 weeks after any dosage change
  • Once stable, monitor every 3-12 months until growth is completed 3

• Pregnant women:

  • Check TSH as soon as pregnancy is confirmed
  • Monitor at minimum once each trimester 1, 3
  • Target TSH range: 0.5-2.0 mIU/L 1

• Central hypothyroidism:

  • Monitor free T4 levels (not TSH)
  • Target free T4 in the upper half of the normal range 1, 3

• Elderly or cardiac patients:

  • Monitor TSH targeting a range of 1.0-4.0 mIU/L 1
  • More frequent monitoring may be needed if clinical status changes

Monitoring When Starting Medications That Affect Levothyroxine

• Additional monitoring is necessary when starting medications that affect levothyroxine absorption or metabolism 1
• Check TSH 4-8 weeks after starting such medications

Clinical Pitfalls and Considerations

• Subclinical hypothyroidism: Many patients are asymptomatic, making it challenging to attribute symptoms to thyroid dysfunction 4
• Medication compliance: Persistent clinical and laboratory evidence of hypothyroidism despite an apparently adequate replacement dose may indicate poor absorption, medication interactions, or non-compliance 3
• Adrenal insufficiency: When both adrenal insufficiency and hypothyroidism are present, steroids must be started before thyroid hormone to avoid precipitating an adrenal crisis 1
• Overtreatment risk: About one-third of patients may be overtreated, highlighting the importance of following guidelines and establishing proper diagnostic criteria 4

Target Laboratory Values

• Standard adults under 70 without cardiac disease:
  • TSH target range: 0.5-2.0 mIU/L 1
• Elderly patients or those with cardiac conditions:
  • TSH target range: 1.0-4.0 mIU/L 1
• Central hypothyroidism:
  • Free T4 target: Upper half of normal range 1

By following these evidence-based guidelines for laboratory testing and monitoring, clinicians can effectively diagnose hypothyroidism and optimize treatment while minimizing risks of under- or over-treatment.