Thyroid Function Interpreter
Interprets TSH and free T4 pattern to classify thyroid function status.
Score interpretation
Normal TSH and normal free T4.
→ No further thyroid testing unless clinical concern persists; TSH alone can mislead in pituitary disease, non-thyroidal illness and recent thyrotoxicosis treatment.
Mildly raised TSH with normal free T4.
→ Repeat TSH and free T4 in 3 months and check TPO antibodies (NICE NG145); consider a 6-month trial of levothyroxine in adults under 65 with symptoms.
TSH above 10 mU/L with normal free T4.
→ Consider levothyroxine if TSH is 10 or higher on 2 tests 3 months apart (NICE NG145).
Raised TSH with low free T4.
→ Start levothyroxine. Adults: 1.6 micrograms/kg/day (rounded to the nearest 25 micrograms) if under 65 without cardiovascular disease; 25–50 micrograms/day, titrated, if 65 or over or with cardiovascular disease (NICE NG145). Children: use a children's formulary. Recheck TSH every 3 months until stable.
Low TSH with normal free T4.
→ Repeat TSH, free T4 and free T3 (T3-toxicosis) in 3 months; consider non-thyroidal illness, drugs (glucocorticoids, dopamine) and levothyroxine over-replacement; refer if TSH stays below 0.1 mU/L. Assess bone and cardiac (AF) risk.
Low TSH with raised free T4.
→ Endocrinology referral; TSH-receptor antibodies (Graves'); beta-blocker for symptoms. Adults: carbimazole 20–40 mg/day (specialist-led). Children: use a children's formulary. Definitive options: radioiodine or thyroidectomy.
Pattern of central (secondary) hypothyroidism — also seen in non-thyroidal illness, after treatment of thyrotoxicosis and with some drugs.
→ Repeat after any acute illness; check 9 am cortisol before starting levothyroxine (risk of adrenal crisis) and assess pituitary function; endocrinology referral.
Discordant pattern — levothyroxine taken before the test or erratic adherence, amiodarone or heparin, assay interference, or rarely a TSH-secreting adenoma or resistance to thyroid hormone.
→ Check the drug history and timing; repeat on a different assay platform; refer to endocrinology if it persists.
Interpretation bands for the TFT Interpreter. Apply clinical judgement and local guidance.
References
- Koulouri O, Gurnell M. How to interpret thyroid function tests. Clin Med 2013;13:282-286.
- Koulouri O et al. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab 2013;27:745-762.
- NICE NG145. Thyroid disease: assessment and management. 2019.
Related
Curated clinical cross-links plus same-class fallbacks.
- Levothyroxine · Thyroid Hormone
- Liothyronine Sodium (T3) · Thyroid Hormone (T3)
- Potassium Iodide / Lugol's Solution · Thyroid Blocking Agent / Pre-operative Thyroid Preparation
- Liothyronine sodium (T3) · Thyroid hormone (T3)
- Levothyroxine sodium · Thyroid hormone (T4)
- Levothyroxine (Elderly) · Thyroid Hormone
- Diabetic Ketoacidosis (DKA) · JBDS 2013 / Joint British Diabetes Societies; NICE NG17
- Adult Hypoglycaemia (Treated Diabetes) · JBDS-IP (2023): Hospital Management of Hypoglycaemia
- Adrenal Crisis · Society for Endocrinology Emergency Guidance (2024)
- Type 2 Diabetes Management · NICE NG28 2022
- Hyperthyroidism Management · BTA / ETA 2018
- Adrenal Insufficiency · Society of Endocrinology / ESE 2016
Decision support only — verify against a current formulary, NICE, or your local guideline before clinical use.