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

Laboratory Tests for Hypothyroidism and Monitoring Frequency

Serum TSH is the primary screening and monitoring test for hypothyroidism, with initial testing followed by free T4 measurement if TSH is abnormal, and monitoring should occur every 6-12 months once stable on treatment. 1

Initial Diagnostic Testing Algorithm

1. Primary screening test: Serum TSH

   • High sensitivity (98%) and specificity (92%) for thyroid dysfunction 2
   • If TSH is elevated, proceed to free T4 testing
   • If TSH is low, measure both free T4 and T3 1, 3

2. Confirmatory testing:

   • Elevated TSH with low free T4: Overt hypothyroidism
   • Elevated TSH with normal free T4: Subclinical hypothyroidism
   • Low/normal TSH with low free T4: Suspect central hypothyroidism 1

3. Additional testing when indicated:

   • Thyroid peroxidase (TPO) antibodies: To confirm autoimmune thyroiditis (Hashimoto's disease) 1
   • Morning cortisol/ACTH: If central hypothyroidism is suspected, especially before starting thyroid hormone replacement 2

Monitoring Frequency

For Patients Starting Treatment:

• Check TSH and free T4 6-8 weeks after initiating treatment or changing dose 1, 4
• For pediatric patients: Check at 2 and 4 weeks after treatment initiation 4

For Stable Patients on Treatment:

• Primary hypothyroidism: Monitor TSH every 6-12 months 1, 4
• Central hypothyroidism: Monitor free T4 levels rather than TSH, targeting the upper half of the normal range 1, 4
• More frequent monitoring (every 3-6 months) for:
  • Pregnant women: At minimum once each trimester 4
  • Children: Every 3-12 months until growth is completed 4
  • Elderly patients or those with cardiac disease: Consider more frequent monitoring during dose adjustments 1

Special Situations Requiring More Frequent Monitoring:

• When starting medications that affect levothyroxine absorption or metabolism (antacids, calcium/iron supplements, proton pump inhibitors, estrogens) 1
• Changes in clinical status or development of new symptoms 4
• Poor compliance suspected 4

Target Laboratory Values

• Most adults: TSH 0.5-2.0 mIU/L 1
• Elderly patients or those with cardiac conditions: TSH 1.0-4.0 mIU/L 1
• Pregnant women: Maintain TSH in trimester-specific reference range 4
• Central hypothyroidism: Target free T4 in upper half of normal range 1, 4

Common Pitfalls and Caveats

• TSH values can be affected by non-thyroidal illness, medications, and pituitary dysfunction 1
• Symptoms of hypothyroidism are non-specific and may overlap with other conditions, making laboratory confirmation essential 5, 6
• When both adrenal insufficiency and hypothyroidism are present, always start steroids before thyroid hormone to avoid precipitating an adrenal crisis 2, 1
• TSH is not a reliable marker in central hypothyroidism; free T4 should be used instead 1, 3
• Subclinical hypothyroidism may not cause symptoms in many patients, making attribution of symptoms challenging 6

By following this structured approach to laboratory testing and monitoring, clinicians can effectively diagnose hypothyroidism and optimize treatment outcomes while minimizing risks associated with under or over-treatment.