Thyroid function tests are blood tests that assess whether the thyroid gland is working normally. They are among the most commonly ordered laboratory tests in medicine, and understanding what they measure and how to interpret them is key to managing thyroid health.
TSH (Thyroid-Stimulating Hormone)
TSH is produced by the pituitary gland and is the most sensitive and important screening test for thyroid dysfunction. It reflects thyroid hormone levels in the body over the past 4–6 weeks. Because TSH and thyroid hormone have an inverse relationship, a high TSH indicates the body is signaling for more thyroid hormone (hypothyroidism), while a low TSH indicates excess hormone (hyperthyroidism).
Normal range: Approximately 0.4–4.0 mIU/L (ranges vary by laboratory and trimester in pregnancy).
Free T4 (Free Thyroxine)
T4 is the main hormone produced by the thyroid. The "free" fraction (unbound to protein) is the biologically active portion. Free T4 is measured when TSH is abnormal to assess severity. Low free T4 with high TSH = overt hypothyroidism. High free T4 with low TSH = overt hyperthyroidism.
Free T3 (Free Triiodothyronine)
T3 is the more potent, active form of thyroid hormone. Most T3 is converted from T4 in the body's tissues. Free T3 testing is most useful in evaluating hyperthyroidism and "T3 toxicosis" (where T3 is elevated but T4 is normal).
💡 Total vs. Free hormones: "Total" T4 and T3 include both bound and free fractions. Since protein levels change in pregnancy, illness, and with certain medications, free T4 and free T3 are generally more reliable for clinical decision-making.
Thyroid Antibodies
| Antibody | What It Indicates |
|---|---|
| TPO Antibodies (TPO-Ab) | Hashimoto's thyroiditis; also elevated in Graves' disease |
| Thyroglobulin Antibodies (TgAb) | Hashimoto's; can interfere with thyroglobulin testing |
| TSH Receptor Antibodies (TRAb / TSI) | Graves' disease; also useful to predict neonatal hyperthyroidism in pregnancy |
Thyroglobulin (Tg)
Thyroglobulin is a protein produced exclusively by thyroid tissue. It is used as a tumor marker in patients who have had thyroid cancer surgery — elevated levels after thyroidectomy may indicate residual or recurrent cancer. It is not used for general thyroid screening.
Calcitonin
Calcitonin is produced by C-cells in the thyroid and is a tumor marker for medullary thyroid cancer. It is measured when medullary cancer is suspected or in patients with a family history of MTC or MEN2 syndrome.
Interpreting Your Results
Lab values must always be interpreted in the context of symptoms, clinical history, and other test results. A single mildly abnormal TSH may reflect transient illness, laboratory variation, or medication effects. Your endocrinologist will interpret your results alongside your complete clinical picture before recommending any changes to treatment.
Key Takeaways
- TSH is the primary screening test — it's the most sensitive indicator of thyroid health
- Free T4 and free T3 provide additional detail when TSH is abnormal
- Thyroid antibody testing identifies autoimmune conditions like Hashimoto's and Graves'
- Thyroglobulin is used for thyroid cancer surveillance after surgery
- Results must always be interpreted in context — never in isolation