What labs are done to evaluate an elevated Thyroid-Stimulating Hormone (TSH) with hypothyroidism?

Laboratory Evaluation for Elevated TSH with Hypothyroidism

The cornerstone laboratory tests for evaluating elevated TSH with hypothyroidism are TSH and free T4, with thyroid autoantibody testing (TPOAb and TgAb) recommended when Hashimoto's thyroiditis is suspected. 1

Initial Laboratory Evaluation

• TSH and free T4: These are the primary tests for evaluating thyroid function, with TSH being the most sensitive indicator of thyroid dysfunction 1, 2

  • TSH is the preferred test for initial evaluation of suspected primary hypothyroidism 2
  • Free T4 confirms the degree of hypothyroidism and helps distinguish between subclinical and overt hypothyroidism 1

• Thyroid autoantibody testing:

  • Thyroid peroxidase antibodies (TPOAb) and thyroglobulin antibodies (TgAb) should be measured when autoimmune thyroiditis (Hashimoto's) is suspected 1
  • Positive antibodies help confirm the etiology and may predict progression from subclinical to overt hypothyroidism 3

Additional Laboratory Tests to Consider

• Free T3 or Total T3: May be helpful in certain situations, particularly when TSH is abnormal but free T4 is normal 2

  • T3 levels are typically ordered less frequently than TSH and T4 for initial hypothyroidism evaluation

• Basic laboratory tests: Complete blood count (CBC), electrolytes, C-reactive protein (CRP), and liver function tests may be considered to evaluate the patient's overall condition 1

Interpretation of Laboratory Results

• Primary hypothyroidism: Elevated TSH with normal or low free T4

  • Subclinical hypothyroidism: TSH above reference range (typically >4.5 mIU/L) with normal free T4 3
  • Overt hypothyroidism: Elevated TSH with low free T4

• Reference ranges:

  • Normal TSH range is typically 0.45-4.5 mIU/L 3
  • Some experts suggest a narrower upper limit of 2.5 mIU/L, though this remains controversial 3

Monitoring Considerations

• Repeat testing: The American College of Physicians recommends confirmation with repeat testing before initiating treatment for abnormal TSH levels 1

• Frequency of monitoring:

  • After initiating treatment: Check TSH and free T4 every 4-6 weeks until stable 1
  • Once stable: Monitor every 3-6 months initially, then annually 1

Common Pitfalls to Avoid

1. Relying solely on TSH without free T4: Both tests are needed to distinguish between subclinical and overt hypothyroidism 1

2. Failure to exclude other causes of elevated TSH:

   • Recent adjustments in levothyroxine dosage
   • Recovery from severe illness
   • Recovery from destructive thyroiditis
   • Untreated primary adrenal insufficiency
   • Heterophilic antibodies causing falsely high TSH 3

3. Misdiagnosis due to assay limitations: Standard immunoassays for free T4 may have poor performance at the upper and lower limits of the reference interval. A significant proportion of patients classified as having subclinical hypothyroidism by immunoassay may actually have low free thyroid hormone levels when measured by more accurate methods like mass spectrometry 4

4. Overdiagnosis: Laboratory reference intervals are based on statistical distribution rather than clinical outcomes 1

5. Inadequate confirmation: Failing to confirm abnormal thyroid function tests before initiating treatment (except in severe cases) 1

By following this structured approach to laboratory evaluation of elevated TSH with hypothyroidism, clinicians can accurately diagnose the condition, determine its severity, and establish appropriate monitoring parameters for treatment.