Thyroid function tests
Also known as: thyroid panel, thyroid blood test, free T4 and TSH
TSH from the pituitary and the thyroid hormones T4 and T3 measured together. Because the pituitary reacts to tiny changes in hormone level, TSH is the most sensitive single indicator of thyroid status.
- Abbreviations
- TFTs, TFT
- Kind
- Test panel
- Categories
- Endocrine & hormone
- Specimen
- Serum
- Method
- Immunoassay
- Component tests
- TSH test
- Type
- Blood test
- Sample
- Blood from a vein
- Used for
- Suspected thyroid disease, monitoring treatment, pituitary disease
- Measures
- TSH with free T4 and sometimes free T3 and thyroid antibodies
- Result
- Numeric values with laboratory reference intervals, read as a pattern
Why is it used?
- Tiredness, weight change, feeling cold or hot, palpitations, anxiety
- A goitre or neck lump
- Atrial fibrillation, high cholesterol, depression
- Monitoring levothyroxine or antithyroid drugs
- Amiodarone or lithium treatment, pregnancy planning
What does it measure?
| Component | What it measures | A higher value may be seen in | A lower value may be seen in |
|---|---|---|---|
| TSH (thyroid-stimulating hormone) TSH TSH | Pituitary drive to the thyroid | Hypothyroidism | Hyperthyroidism, pituitary disease |
| Free T4 (thyroxine) FT4 Free T4 | Unbound thyroxine | Hyperthyroidism, excess levothyroxine | Hypothyroidism, pituitary disease, severe illness |
| Free T3 FT3 | Unbound triiodothyronine, the active hormone | Hyperthyroidism (T3 toxicosis) | Severe illness, hypothyroidism (late) |
| Thyroid peroxidase antibodies (when requested) TPO | Autoimmune thyroid disease | Hashimoto thyroiditis, Graves disease; predicts progression of subclinical hypothyroidism | Not applicable |
A high or low value can be associated with several different situations; the result is interpreted with other results, symptoms, history and clinical findings.
How is it performed?
Venous blood, any time of day (take levothyroxine after the sample).
Specimen. Venous blood (serum)
Do I need to prepare?
None; biotin supplements can interfere and should be paused for two days.
What do the results generally mean?
| Finding | What it may indicate |
|---|---|
| High TSH, low T4 | Primary hypothyroidism |
| High TSH, normal T4 | Subclinical hypothyroidism; treat if symptomatic, pregnant or TSH very high |
| Low TSH, high T4 or T3 | Hyperthyroidism: Graves disease, toxic nodules, thyroiditis |
| Low TSH and low T4 | Pituitary (secondary) failure, or recent overtreatment |
What can affect the result?
- Acute and severe illness (sick euthyroid pattern)
- Biotin supplements interfering with the assay
- Pregnancy and the first trimester rise in hCG
- Amiodarone, lithium, glucocorticoids, dopamine, some antiepileptics
- Levothyroxine timing and absorption
Medicines. Levothyroxine and antithyroid drugs are the intended influences; amiodarone, lithium, interferons and immune checkpoint inhibitors can cause thyroid dysfunction; iron, calcium and proton pump inhibitors reduce levothyroxine absorption.
What can the test not tell you?
- The cause of thyroid dysfunction without antibodies, ultrasound or uptake scans
- Whether a nodule is present or cancerous
- Thyroid status reliably during severe non-thyroidal illness
Acute illness and some drugs disturb results ('sick euthyroid'); repeat once recovered. TSH lags six weeks behind a dose change.
Other tests commonly used with it
Medicines monitored with this test
Each medicine page cites the official document that names this test in its monitoring plan.
Catalogued concepts this page covers
Concepts of the master test taxonomy that resolve to this page; each links to its category.