How to Read a Thyroid Blood Test

Thyroid results are interpreted by looking at TSH, free T4, and free T3 together.

The Three Key Tests

• TSH: pituitary signal to thyroid (0.4–4.0 mIU/L)
• Free T4: main hormone produced by thyroid (0.8–1.8 ng/dL)
• Free T3: active hormone (2.3–4.2 pg/mL)

Common Patterns

• High TSH + Low Free T4 = primary hypothyroidism
• High TSH + Normal Free T4 = subclinical hypothyroidism
• Low TSH + High Free T4 = hyperthyroidism
• Low TSH + Normal Free T4 = subclinical hyperthyroidism
• Low TSH + Low Free T4 = central hypothyroidism (pituitary)
• Normal TSH + Normal T4 = normal thyroid function

Adding Antibodies

• Anti-TPO positive + high TSH = Hashimoto's
• TSI positive + low TSH = Graves' disease

Tips

• Always test at the same time of day
• Stop biotin supplements 3 days before testing
• Interpret trends, not single values