Thyroid Function Test Interpreter
mIU/L (normal: 0.4–4.0)
ng/dL (normal: 0.8–1.8)
pg/mL (normal: 2.3–4.2)
affects reference range interpretation
—
—
TSH
—
mIU/L
Free T4
—
ng/dL
Free T3
—
pg/mL
Thyroid Function Pattern Recognition
| TSH | Free T4 | Free T3 | Interpretation |
|---|---|---|---|
| Normal (0.4–4) | Normal | Normal | Euthyroid — Normal thyroid function |
| High (>4) | Low (<0.8) | Low / Normal | Overt Hypothyroidism — treat with levothyroxine |
| High (>4–10) | Normal | Normal | Subclinical Hypothyroidism — consider treatment |
| Very High (>10) | Normal or Low | — | Significant Hypothyroidism — treat |
| Low (<0.4) | High (>1.8) | High (>4.2) | Overt Hyperthyroidism — antithyroid drugs / RAI |
| Low (<0.4) | Normal | Normal | Subclinical Hyperthyroidism — monitor / treat if symptomatic |
| Low | Low | Low | Central Hypothyroidism — pituitary/hypothalamic cause |
| Low or Normal | Low | Very Low | Sick Euthyroid Syndrome — non-thyroidal illness |
Levothyroxine (T4) Replacement Dosing
- Young healthy adults (overt hypothyroidism): Full replacement dose — 1.6 µg/kg/day. Start at full dose in healthy adults under 60
- Elderly or cardiac disease: Start low — 25–50 µg/day, increase by 25 µg every 4–8 weeks. Avoid precipitating angina or AF
- Subclinical hypothyroidism (TSH 4–10, normal FT4): 50 µg/day starting dose if treating. Repeat TFT in 6–8 weeks. Target TSH 0.5–2.5 mIU/L for most adults; 0.5–1.5 in younger patients; can accept 1–4 in elderly
- Pregnancy: Increase levothyroxine dose immediately by 25–30% on diagnosis of pregnancy. Target TSH <2.5 mIU/L in first trimester, <3 mIU/L in second and third trimester
- Monitoring: Recheck TFT 6–8 weeks after any dose change. Once stable, annual TSH monitoring
Causes of Hypothyroidism
- Hashimoto thyroiditis (autoimmune): Most common cause in iodine-sufficient countries. Anti-TPO antibodies positive in 95%. Gradual destruction of thyroid tissue
- Iodine deficiency: Most common cause globally. Endemic in inland mountainous regions and certain parts of South Asia and Africa
- Post-radioactive iodine (RAI) or thyroidectomy
- Drug-induced: Amiodarone (iodine content), lithium (inhibits thyroid hormone release), interferon-α, checkpoint inhibitors
- Central (secondary/tertiary): Pituitary or hypothalamic disease — low TSH with low FT4
Causes of Hyperthyroidism
- Graves disease: Most common cause. Autoimmune stimulation of TSH receptor by TSH-receptor antibodies (TRAb). Diffuse goitre, ophthalmopathy, pretibial myxoedema
- Toxic nodular goitre (Plummer disease): Single or multiple autonomously functioning nodules — common in iodine-deficient areas and elderly
- Subacute (de Quervain) thyroiditis: Post-viral painful thyroiditis with transient hyperthyroid phase, followed by hypothyroid phase, then recovery. Low uptake on thyroid scan
- Exogenous thyroid hormone (factitious): Low thyroglobulin level distinguishes from intrinsic overproduction
- Amiodarone-induced thyrotoxicosis (AIT): Type 1 (excess iodine) or Type 2 (destructive thyroiditis)
Frequently Asked Questions
Related Calculators
⚠ Medical Disclaimer: TSH and thyroid function interpretation requires clinical correlation with symptoms, history, medication use, and repeat testing where appropriate. Reference ranges vary by laboratory and assay. Pregnancy, acute illness, medications (amiodarone, biotin, heparin), and assay interference can significantly affect results. Always interpret TFTs in the clinical context. Endocrinology referral for complex or uncertain cases.