What blood tests are used to diagnose hyperthyroidism?

Blood Tests for Diagnosing Hyperthyroidism

The primary blood test for diagnosing hyperthyroidism is serum thyroid-stimulating hormone (TSH), followed by free thyroxine (FT4) and free triiodothyronine (FT3) measurements when TSH is abnormal. 1

First-Line Tests

• Serum TSH is the most sensitive initial screening test for thyroid dysfunction, with a sensitivity of approximately 98% and specificity of 92% when used to confirm suspected thyroid disease 2
• Low TSH levels (typically below 0.1 mIU/L) suggest hyperthyroidism 2
• When TSH is abnormal, free thyroxine (FT4) and/or free triiodothyronine (FT3) measurements should be performed to differentiate between subclinical hyperthyroidism (normal FT4/FT3) and overt hyperthyroidism (elevated FT4/FT3) 3, 1
• Multiple TSH measurements should be done over a 3-6 month interval to confirm abnormal findings, as TSH levels can vary by up to 50% day-to-day 2, 3

Additional Tests

• TSH-receptor antibodies (TRAb) should be measured to confirm Graves' disease, which is the most common cause of hyperthyroidism (approximately 70% of cases) 4
• Thyroid peroxidase antibodies (anti-TPO) may be helpful in identifying autoimmune thyroid disease 3, 4
• Free T3 (FT3) measurement is particularly important for diagnosing hyperthyroidism, as some patients may have "T3-toxicosis" with normal T4 but elevated T3 levels 5

Interpretation of Results

• Hyperthyroidism is biochemically confirmed by low TSH with elevated FT4 and/or FT3 4
• Subclinical hyperthyroidism is characterized by low TSH with normal FT4 and FT3 levels 2, 4
• Using a TSH cutoff value of 0.1 mIU/L provides a sensitivity of 98% and specificity of 98% for diagnosing hyperthyroidism 6

Important Considerations

• TSH secretion can be affected by factors other than thyroid disorders, including:
  • Acute illness (can suppress TSH) 2
  • Medications (iodine, dopamine, glucocorticoids, octreotide, bexarotene) 2, 3
  • Pregnancy (particularly first trimester) 2
  • Adrenal insufficiency, anorexia nervosa, certain autoimmune diseases, and pituitary adenomas 2
• TSH reference ranges vary among different laboratories (generally about 0.4 to 4.5 mIU/L) 2
• TSH secretion varies among different subpopulations based on race/ethnicity, sex, and age 2

Common Pitfalls to Avoid

• Relying on a single TSH value for diagnosis - serial measurements are essential due to day-to-day variability 2, 3
• Ordering unnecessary tests - when TSH levels are in the reference range, additional tests like FT4, FT3, or thyroid antibodies rarely add value (except in patients with pituitary disease) 1
• Overlooking the possibility of T3-toxicosis - some patients may have hyperthyroidism with normal T4 but elevated T3 levels 5
• Failing to consider non-thyroidal factors that can affect TSH levels 2, 3
• Overzealous use of thyroid ultrasound - it is not part of routine assessment of hyperthyroidism and can lead to identification of clinically unimportant thyroid nodules 1

Testing Algorithm

1. Measure serum TSH as the initial screening test 1
2. If TSH is low (< 0.1 mIU/L), measure FT4 and FT3 4, 1
3. If FT4 and/or FT3 are elevated, overt hyperthyroidism is confirmed 4
4. If FT4 and FT3 are normal with low TSH, subclinical hyperthyroidism is present 2
5. Measure TSH-receptor antibodies to determine if Graves' disease is the cause 4
6. Repeat TSH measurement after 3-6 months to confirm persistence of abnormal results 2, 3