Thyroid Function Test — Test Guide & Results Explained — Purpose, Procedure & Normal Values | MyMedicPlus

Thyroid function tests (TFTs) are a panel of blood tests that assess the activity and health of the thyroid gland — a butterfly-shaped endocrine gland in the front of the neck that regulates the body's metabolism, energy production, heart rate, body temperature, growth, and development through secretion of thyroid hormones. The key tests included in a standard thyroid function panel are: TSH (thyroid-stimulating hormone) — a pituitary hormone that acts as the primary regulator of thyroid output and is the most sensitive first-line marker of thyroid dysfunction (TSH is elevated in hypothyroidism and suppressed in hyperthyroidism); Free T4 (free thyroxine) — the main hormone secreted directly by the thyroid gland, reflecting the gland's actual output; Free T3 (free triiodothyronine) — the biologically active form, predominantly converted from T4 in peripheral tissues, with direct effects on target organs. Thyroid antibodies — anti-thyroid peroxidase antibodies (anti-TPO), anti-thyroglobulin antibodies (anti-Tg), and TSH receptor antibodies (TRAb) — are added to the panel when autoimmune thyroid disease is clinically suspected (Hashimoto's thyroiditis, Graves' disease). Reverse T3, thyroid ultrasound, and radioiodine uptake scans may be used for additional investigation.

Thyroid function tests are ordered to investigate symptoms of hypothyroidism — fatigue, unexplained weight gain, cold intolerance, constipation, dry skin, brittle hair, hair loss, depression, impaired memory and concentration, bradycardia (slow heart rate), puffiness of the face and hands, and low mood. They are equally ordered to investigate symptoms of hyperthyroidism — unexplained weight loss despite increased appetite, palpitations, tremor, heat intolerance and excessive sweating, diarrhoea, anxiety, agitation, atrial fibrillation, exophthalmos (bulging eyes in Graves' disease), and a visible or palpable thyroid enlargement (goitre). TSH is included in antenatal screening because maternal thyroid dysfunction — both hypothyroidism and hyperthyroidism — is associated with adverse pregnancy outcomes including miscarriage, preterm birth, and impaired fetal neurodevelopment. TFTs are used to monitor the adequacy and dose accuracy of levothyroxine replacement therapy in treated hypothyroidism, to monitor the response to antithyroid drug therapy (carbimazole, propylthiouracil) or to radioiodine ablation in hyperthyroidism, to evaluate a thyroid nodule or enlarging goitre, and to screen neonates for congenital hypothyroidism (heel-prick screening).

No fasting is required for a thyroid function test. However, testing consistently at the same time of day — preferably in the morning between 8 and 10 AM — improves the comparability of serial results over time, because TSH exhibits a clinically meaningful diurnal variation: it peaks between midnight and 4 AM and reaches its lowest point in the afternoon. For patients already taking levothyroxine replacement therapy, take the morning levothyroxine dose after the blood draw rather than before, to avoid a transient post-dose peak in free T4 that does not reflect the true steady-state level. Inform your doctor if you are taking high-dose biotin supplements (vitamin B7 above 5–10 mg daily), as biotin at pharmacological doses causes significant analytical interference in many immunoassay platforms, falsely elevating free T4 and free T3 results and falsely suppressing TSH — which can mimic hyperthyroidism; stop high-dose biotin for at least 48–72 hours before thyroid testing. Acute or severe illness (non-thyroidal illness syndrome, also called euthyroid sick syndrome) can temporarily alter TFT results even in people with a completely healthy thyroid gland — defer routine testing during acute hospital admissions where possible.

A phlebotomist draws 3–5 mL of venous blood from the antecubital fossa of the arm using a standard venepuncture technique. The blood is collected into a serum separator tube (gold or yellow top), which contains a clot activator and gel separator. After collection, the tube is gently inverted 5–10 times and allowed to clot for 20–30 minutes before centrifugation separates the serum from the cellular components. The serum is transferred and analysed on an automated immunoassay platform — typically an electrochemiluminescent immunoassay (ECLIA) or chemiluminescent microparticle immunoassay (CMIA) — which measures TSH, free T4, and free T3 by competitive or sandwich immunoassay principles. Thyroid antibody tests (anti-TPO, anti-Tg, TRAb) use the same analytical platform. Results for a basic TSH and free T4 panel are routinely available within 4–8 hours; a full antibody panel may take 24–48 hours depending on laboratory batch scheduling. The entire blood draw appointment takes less than 10 minutes.

Normal reference ranges vary slightly between laboratories and analysers. Widely accepted adult reference ranges are: TSH: 0.4–4.0 mIU/L (some laboratories use 0.27–4.2 mIU/L); Free T4: 0.8–1.8 ng/dL (10–23 pmol/L); Free T3: 2.3–4.1 pg/mL (3.5–6.5 pmol/L). Clinical interpretation patterns: High TSH with low free T4 — primary hypothyroidism (thyroid gland underactive, requiring treatment); Low TSH with high free T4 or free T3 — overt hyperthyroidism (Graves' disease, toxic nodular goitre, thyroiditis); Low TSH with normal free T4 — subclinical hyperthyroidism (monitor or treat if symptomatic); High TSH with normal free T4 — subclinical hypothyroidism (treat if TSH above 10 mIU/L or if symptomatic with lower TSH elevation); Normal TSH — euthyroid, no thyroid dysfunction. Anti-TPO antibodies positive: present in more than 90% of Hashimoto's thyroiditis cases and 70–80% of Graves' disease; a positive result does not always mean treatment is needed but indicates autoimmune thyroid disease. TRAb (TSH receptor antibodies) positive specifically indicates Graves' disease, distinguishing it from other causes of hyperthyroidism.

The blood draw carries minimal risks — occasional minor bruising, haematoma at the venepuncture site, or a brief vasovagal (fainting) reaction are the only realistic complications. The main analytical and clinical limitations of thyroid function testing are: Non-thyroidal illness syndrome (euthyroid sick syndrome) — acute or chronic severe illness suppresses TSH and T3, and alters T4, independent of true thyroid disease; interpreting TFTs in critically ill patients requires specialist input. Biotin interference — pharmacological biotin doses (used for hair/nail supplements) cause spuriously abnormal TFT results in many immunoassay systems; patients should withhold biotin for 48–72 hours before testing. Medication interference — amiodarone (contains 37% iodine by weight) raises T4 and can suppress TSH; lithium reduces thyroid hormone secretion and raises TSH; corticosteroids and dopamine suppress TSH; heparin displaces T4 from binding proteins; thyroid hormone assays can be affected by heterophilic antibodies in some patients. TSH does not accurately reflect thyroid status during pregnancy (lower TSH is normal in the first trimester), in central hypothyroidism (pituitary disease), or immediately after a change in levothyroxine dose — it takes 6–8 weeks for TSH to reach a new steady state.