Key points
Start with TSH. The American Association of Clinical Endocrinologists (AACE) and the American Thyroid Association (ATA) call thyroid-stimulating hormone (TSH) the best single test for detecting primary (arising in the gland itself) thyroid disorders in most outpatient settings (Garber JR Thyroid 2012. PMID 22954017); with pituitary disease TSH may be normal (Soh SB Ann Lab Med 2019. PMID 30215224).
Free T4 complements TSH. When TSH is out of range, it is not enough on its own: free thyroxine (T4) is assessed alongside it to tell an overt problem from a mild one (Soh SB Ann Lab Med 2019. PMID 30215224).
Antibodies show the cause. Thyroid peroxidase antibodies point to Hashimoto's thyroiditis, TSH-receptor antibodies to Graves' disease (Soh SB Ann Lab Med 2019. PMID 30215224).
Ultrasound — when there is a nodule. An ultrasound scan is a mandatory step when a nodule is found, while looking for thyroid cancer in adults without symptoms and without increased risk (radiation exposure, including radioactive fallout, or thyroid cancer in close relatives) is not recommended (Russ G Eur Thyroid J 2017. PMID 29167761; USPSTF JAMA 2017. PMID 28492905).
When to get your thyroid checked
Symptoms of a thyroid problem are non-specific: tiredness, feeling cold or sweating, weight change, palpitations, hair loss, menstrual cycle disturbances, difficulty conceiving. Iron deficiency, lack of sleep or stress cause the same complaints, so tests look for the cause rather than treat the symptom. It is also worth getting checked if you have autoimmune diseases.
TSH — the first test
TSH is made by the pituitary gland. When thyroid hormones are lacking, the pituitary raises TSH; when there are too many, it lowers it. That is why TSH responds to shifts earlier than the hormones themselves.
A typical reference range is roughly 0.4–4.0 mIU/L (0.4–4.0 µIU/mL): the European Thyroid Association (ETA) considers a level from 4.0 mIU/L (4.0 µIU/mL) upwards a mild elevation (Pearce SH Eur Thyroid J 2013. PMID 24783053). Each laboratory has its own limits, and in older people the upper limit is higher (Soh SB Ann Lab Med 2019. PMID 30215224).
One abnormal result is not yet a diagnosis. If TSH is raised but free T4 is normal, the ETA advises repeating both tests together with thyroid peroxidase antibodies, preferably after 2–3 months (Pearce SH Eur Thyroid J 2013. PMID 24783053). The doctor assesses a series of results, not a single point.
Free T4 and when triiodothyronine (T3) is needed
Free T4 is the main thyroid hormone in its free form, not bound to blood proteins. In Ukraine and Europe it is measured in pmol/L, in the US in ng/dL: for example, 15.4 pmol/L (1.2 ng/dL). The reference range depends on the analyser — check the limits on your lab report.
Together, TSH and free T4 give the main patterns: TSH raised and T4 low — hypothyroidism; TSH raised and T4 normal — subclinical (hidden) hypothyroidism; TSH low and T4 raised — hyperthyroidism (Soh SB Ann Lab Med 2019. PMID 30215224).
Free T3 is not usually the first test. It is needed when TSH is low but free T4 is normal: this rules out a form of hyperthyroidism in which only T3 is raised. The T3 test is also less accurate than the T4 test (Soh SB Ann Lab Med 2019. PMID 30215224).
Antibodies: what they mean
Thyroid peroxidase antibodies (TPOAb). The main marker of Hashimoto's thyroiditis (autoimmune thyroiditis): in it TPOAb are almost always raised, but they are also found in 5–20% of people overall (Soh SB Ann Lab Med 2019. PMID 30215224). Antibodies alone are not a diagnosis but a reason to keep an eye on TSH. In a 20-year follow-up, women with antibodies only had 8 times higher odds of developing hypothyroidism, and those with antibodies and raised TSH 38 times higher (Vanderpump MP Clin Endocrinol 1995. PMID 7641412). More — on TPOAb.
Thyroglobulin antibodies (TgAb). Raised in about 10% of people, and without TPOAb they are not meaningfully linked to thyroid disease. Their main use is after treatment of differentiated thyroid cancer, to assess thyroglobulin correctly (Soh SB Ann Lab Med 2019. PMID 30215224).
TSH-receptor antibodies (TRAb). Healthy people do not have them, so they are specific to Graves' disease — autoimmune hyperthyroidism (Soh SB Ann Lab Med 2019. PMID 30215224). The ETA recommends this test for an accurate diagnosis, before stopping antithyroid drugs and during pregnancy in women with Graves' disease (Kahaly GJ Eur Thyroid J 2018. PMID 30283735).
Antibodies and pregnancy
With normal thyroid function, antibodies were associated with higher odds of miscarriage (3.9 times in cohort studies) and preterm birth (2.07 times) (Thangaratinam S BMJ 2011. PMID 21558126). For pregnant women with normal thyroid function and TPOAb or TgAb, the ATA advises checking TSH as soon as pregnancy is confirmed and then every 4 weeks until mid-pregnancy (Alexander EK Thyroid 2017. PMID 28056690). TSH targets in pregnancy are different — see TSH norms in pregnancy.
Whether to prescribe levothyroxine for antibodies alone is debatable: in two small studies it reduced the miscarriage rate (Thangaratinam S BMJ 2011. PMID 21558126), but in the large TABLET trial (952 women with TPOAb and miscarriages or infertility) 50 µg a day did not increase the proportion of live births: 37.4% vs 37.9% with placebo (Dhillon-Smith RK N Engl J Med 2019. PMID 30907987).
Ultrasound: when it is needed and when it is not
Ultrasound shows the structure of the gland, not how it works. According to the ETA guideline, it is mandatory when there is a nodule: the ultrasound features decide whether a biopsy is needed (Russ G Eur Thyroid J 2017. PMID 29167761).
The US Preventive Services Task Force (USPSTF) does not recommend looking for thyroid cancer with ultrasound in adults without symptoms and without increased risk (radiation exposure, including radioactive fallout, or thyroid cancer in close relatives): the benefit is at most small, while the harm from overdiagnosis and unnecessary treatment is at least moderate (USPSTF JAMA 2017. PMID 28492905).
How to prepare for the tests
In the morning, at the same hour. TSH changes over the day: it is lowest towards evening and highest between midnight and 4 a.m. (Soh SB Ann Lab Med 2019. PMID 30215224). So have your blood taken in the morning and each time at roughly the same hour.
A break from biotin. Supplements “for hair and nails” can contain up to 10 mg of biotin per tablet (Soh SB Ann Lab Med 2019. PMID 30215224); it can distort TSH, T4 and antibody results, even to the point of a false picture of Graves' disease. The author of one case report with megadoses believes a break of at least 2 days should be enough to avoid a gross error (Barbesino G Thyroid 2016. PMID 27043844). More — on biotin and lab tests.
The same laboratory. Different analysers have different TSH reference ranges (Soh SB Ann Lab Med 2019. PMID 30215224). Do repeat tests at the same place so you see the trend, not the difference between methods.
Which test shows what
| Test | What it shows | When it is needed |
|---|---|---|
| TSH | The pituitary signal: rises when hormones are lacking, falls when there is an excess | First, when a problem is suspected, and to monitor treatment |
| Free T4 | How much free thyroxine, not bound to proteins, there is | Together with TSH when TSH is out of range or there are symptoms |
| Free T3 | The gland's second hormone | When TSH is low but free T4 is normal |
| TPOAb | An autoimmune process, Hashimoto's thyroiditis | When TSH is raised; in other cases — at the doctor's discretion |
| TgAb | An autoimmune process, less specifically | Mostly after treatment of differentiated thyroid cancer |
| TRAb | Graves' disease | When TSH is low and there are signs of hyperthyroidism; in pregnant women with Graves' disease |
| Ultrasound | The structure of the gland, nodules | When a nodule can be felt or the shape of the neck has changed |
Who should be more careful
- If you develop palpitations, trembling hands, unexplained weight loss, or your eyes start to bulge, see a doctor without delay: these may be signs of hyperthyroidism.
- If a nodule in the neck grows quickly, your voice changes or swallowing becomes difficult, see a doctor soon.
- If you are planning a pregnancy or are already pregnant, discuss with your doctor whether you need a TSH test: it is not recommended for everyone without exception (Alexander EK Thyroid 2017. PMID 28056690).
Summary
Thyroid testing usually starts with TSH; free T4, less often T3, and antibodies explain what has gone wrong and why. Ultrasound is needed when there is a nodule. A morning sample, a break from biotin and the same laboratory make the result more accurate. You can go through your results and draw up a testing plan at a consultation.
This material is for information only and does not replace a consultation with a doctor. Which tests you need and how to interpret them is for your doctor to decide.
