What specific blood work is recommended for diagnosing hyperthyroidism?

Specific Blood Work for Diagnosing Hyperthyroidism

The initial diagnostic test for suspected hyperthyroidism should be a thyroid-stimulating hormone (TSH) level, followed by free thyroxine (FT4) and free triiodothyronine (FT3) if TSH is low. 1, 2, 3

Recommended Diagnostic Algorithm

First-Line Testing

1. TSH measurement

   • High sensitivity (98%) and specificity (92%) for thyroid dysfunction 1
   • Values below 0.1 mU/L are considered low and suggest hyperthyroidism 1
   • This is the most efficient initial screening test 3

2. If TSH is low, proceed with:

   • Free T4 (FT4) measurement
   • Free T3 (FT3) measurement if TSH is suppressed but FT4 is normal 2, 3

Diagnostic Interpretation

• Overt hyperthyroidism: Low TSH with elevated FT4 and/or FT3 1, 2, 3
• Subclinical hyperthyroidism: Low TSH with normal FT4 and FT3 1, 3
• T3 toxicosis: Low TSH, normal FT4, elevated FT3 2

Etiological Diagnosis

After confirming biochemical hyperthyroidism, additional testing is needed to determine the underlying cause:

1. TSH-receptor antibodies (TRAb)

   • Positive in Graves' disease (the most common cause of hyperthyroidism) 2, 3

2. Thyroid peroxidase antibodies (TPOAb)

   • May be present in autoimmune thyroid disorders 2

3. Thyroid ultrasonography

   • Helps identify nodules, goiter, or inflammatory changes 2

4. Thyroid scintigraphy (radioiodine uptake scan)

   • Recommended when thyroid nodules are present or etiology is unclear 3
   • Helps distinguish between Graves' disease (diffuse uptake), toxic nodular goiter (focal uptake), and thyroiditis (low uptake) 4, 2

Clinical Considerations

• The diagnostic approach should follow a stepwise pattern, starting with TSH and proceeding to additional tests based on results
• A TSH cutoff value of 0.1 mU/L provides 98% sensitivity and 98% specificity for hyperthyroidism 5
• Thyroid function tests may be affected by certain medications, so a thorough medication review is essential
• Special consideration is needed for pregnant patients, as reference ranges differ during pregnancy 2

Common Pitfalls to Avoid

• Relying solely on TSH: While TSH is an excellent screening test, both free T4 and free T3 are needed to properly classify the severity and type of hyperthyroidism
• Missing T3 toxicosis: Some patients may have normal FT4 but elevated FT3, so measuring only TSH and FT4 could miss this condition
• Failing to identify the underlying cause: Determining the etiology is crucial for proper treatment selection, as management differs for Graves' disease, toxic nodular goiter, and thyroiditis 4, 2, 3
• Not considering medication effects: Amiodarone, tyrosine kinase inhibitors, and immune checkpoint inhibitors can cause hyperthyroidism 2

The diagnostic workup for hyperthyroidism should be systematic and thorough to ensure accurate diagnosis and appropriate treatment selection.