Thyroid function tests

Interpretation

A blood test of the thyroid's control hormone (TSH) and the thyroid hormones themselves shows whether the gland is working too much, too little or normally.

Updated Nov 13, 2026

What it is

Thyroid function tests are read as a pair: thyroid-stimulating hormone (TSH) from the pituitary and free thyroxine (free T4) from the thyroid, with free triiodothyronine (free T3) added when thyrotoxicosis is suspected. Because the pituitary answers small changes in thyroid hormone with large opposite changes in TSH, TSH is the most sensitive single screening test when the hypothalamic–pituitary axis is intact. A high TSH with a low free T4 means primary hypothyroidism; a low TSH with a high free T4 or free T3 means primary hyperthyroidism or another cause of thyrotoxicosis. When TSH and free T4 move in the same direction, think of the pituitary (central hypothyroidism, a TSH-secreting adenoma), thyroid hormone resistance, a recent change in treatment, or an assay problem such as biotin or heterophile antibodies. Free hormones are measured rather than total hormones because binding proteins change with pregnancy, oestrogens, liver disease and illness. Acute illness itself distorts the results (non-thyroidal illness), so tests in the critically ill are interpreted with caution or repeated after recovery. Antibodies (anti-thyroid peroxidase, TSH-receptor antibodies) and imaging then point to the cause.

Reference values and concepts

Usual adult TSH: about 0.4–4.0 mIU/L (ranges vary by laboratory and assay, and rise slightly with age).
Usual adult free T4: about 0.8–1.8 ng/dL (roughly 10–23 pmol/L); free T3: about 2.3–4.2 pg/mL.
The TSH–free T4 relationship is log-linear: a small fall in free T4 produces a large rise in TSH.
After a change in thyroid status or levothyroxine, TSH takes about 6–8 weeks to reach a new steady state.
In pregnancy, hCG stimulates the thyroid and lowers TSH in the first trimester; use trimester-specific ranges.
Anti-TPO antibodies support autoimmune (Hashimoto) thyroiditis; TSH-receptor antibodies support Graves disease.

Reading it step by step

Ask why the test was ordered and about drugs, biotin, pregnancy and acute illness — they change the result or its meaning.
Read TSH first and compare it with the laboratory range — it is the most sensitive marker of primary thyroid disease.
Read free T4 next and decide if it moves opposite to TSH — opposite movement means the problem is in the thyroid.
If TSH is low, check free T3 when free T4 is normal — early or nodular thyrotoxicosis may raise only T3.
If TSH and free T4 move the same way, think pituitary, recent treatment change or assay interference — not primary disease.
Look at the clinical picture: symptoms, pulse, goitre, eye signs — a mild abnormality in a well person may need only a repeat.
In hospitalised or very ill patients, consider non-thyroidal illness — low T3 with normal TSH is common and is not hypothyroidism.
Order antibodies or imaging to find the cause — the cause decides treatment, not the numbers alone.

TSH and free T4 patterns (adult; free T3 helps when TSH is low)

TSH	Free T4 / free T3	Likely meaning	Think of
High	Low free T4	Primary hypothyroidism	Hashimoto thyroiditis, after radioiodine or surgery, iodine deficiency, drugs
High	Normal	Subclinical hypothyroidism	Early autoimmune thyroiditis, recovery phase of thyroiditis, missed doses
Low	High free T4 and/or free T3	Primary thyrotoxicosis	Graves disease, toxic nodule or multinodular goitre, thyroiditis, excess thyroid hormone
Low	Normal free T4, high free T3	T3 toxicosis	Early Graves disease, toxic nodule
Low	Normal free T4 and free T3	Subclinical hyperthyroidism	Nodular goitre, over-replacement, first trimester of pregnancy, recovery from illness
Low or normal	Low free T4	Central hypothyroidism or non-thyroidal illness	Pituitary or hypothalamic disease, critical illness, glucocorticoids
Normal or high	High free T4	Inappropriate TSH	TSH-secreting adenoma, thyroid hormone resistance, assay interference, erratic levothyroxine

Patterns and common mistakes

Treating a single mildly abnormal TSH without repeating it in 6–12 weeks: transient changes are common, especially after illness.
Missing central hypothyroidism because only TSH was ordered in a patient with pituitary disease or after pituitary surgery.
Biotin or heterophile antibodies giving results that do not fit the patient; discuss with the laboratory and use another assay.
Erratic levothyroxine intake: a high TSH with a normal or high free T4 often means tablets were taken just before the test.
Amiodarone and lithium can cause hypothyroidism, and amiodarone thyrotoxicosis; check thyroid tests before and during treatment.
Reading total T4 in pregnancy or on oral oestrogen: binding globulin rises and total T4 looks high while free T4 is normal.

Next step

Repeat a mildly abnormal result in 6–12 weeks before labelling disease, unless the patient is unwell or pregnant.
Thyrotoxicosis: send TSH-receptor antibodies and consider an uptake scan or ultrasound to separate Graves, nodules and thyroiditis.
Hypothyroidism: anti-TPO antibodies support autoimmune disease; on levothyroxine recheck TSH about 6–8 weeks after any change.
Low free T4 with a low or normal TSH: assess the other pituitary hormones, especially cortisol, and image the pituitary.
Call a senior urgently for suspected thyroid storm or myxoedema coma: confusion, fever or hypothermia, arrhythmia, shock.
Results that do not fit the patient: ask about biotin and discuss with the laboratory before acting.

Test yourself

Think of the answer, then tap the question to see it.

A tired woman has TSH 18 mIU/L and a low free T4. What is the pattern and the most likely cause?

Primary hypothyroidism: the thyroid fails and the pituitary raises TSH. Autoimmune (Hashimoto) thyroiditis is the commonest cause where iodine intake is adequate; anti-TPO antibodies support it.

After pituitary surgery a man has a low free T4 and a TSH within the reference range. Is his thyroid axis normal?

No. A normal TSH is inappropriate when free T4 is low: this is central hypothyroidism. TSH alone would miss it, so free T4 is followed instead of TSH.

A patient has low TSH and high free T4 but feels well and takes a hair and nail supplement. What should you suspect?

Biotin interference in biotin–streptavidin immunoassays, which can mimic thyrotoxicosis. Ask about supplements, repeat after stopping them for a few days or use another assay.

Why is a low T3 with normal TSH common in intensive care patients?

Acute illness reduces conversion of T4 to T3 and alters TSH and binding proteins (non-thyroidal illness). It is usually not treated; tests are repeated after recovery if needed.

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Laboratory Assessment of Thyroid Function (StatPearls (NCBI Bookshelf), 2025).

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