A thyroid function test is a blood test that checks how well your thyroid is working. It usually starts with thyroid-stimulating hormone (TSH). If TSH is outside the reference range, many labs add free thyroxine (FT4) to the same sample, and sometimes free triiodothyronine (FT3). Thyroid antibody tests are a separate step, used to look for an autoimmune cause.
Doctors often check thyroid function when someone has low mood, tiredness, anxiety or poor concentration. Thyroid problems can cause all of these. But NICE, which writes guidance for England, notes that one symptom alone may not point to thyroid disease. This guide explains each part of the test, common result patterns, why repeat tests are so common, and the test's limits.
If you want to know how thyroid hormone affects thinking and mood, thyroid and the brain covers that. This article is about the test itself.
The pituitary gland, at the base of the brain, makes TSH. TSH tells the thyroid to make more hormone. The British Thyroid Foundation compares the system to a thermostat. When thyroid hormone levels fall, the pituitary releases more TSH. When they rise, it releases less.
The thyroid's main hormone is thyroxine (T4). It is converted to triiodothyronine (T3), the biologically active form. Labs usually measure the "free" portion of each, written FT4 and FT3.
NICE suggests measuring TSH alone in most adults, unless a pituitary problem is suspected. If TSH is above the range, FT4 is measured in the same sample. If TSH is below the range, FT4 and FT3 are measured. The British Thyroid Foundation says many labs use this "cascade" system. It also says that if TSH is normal, it is very likely your thyroid is working normally.
The HSE says a GP may check T3, but that this is not routine. MedlinePlus, from the US National Library of Medicine, says T3 is not commonly used to check for an underactive thyroid.
An underactive thyroid (hypothyroidism) can cause tiredness, depression, feeling cold, weight gain, dry skin, constipation and heavy or irregular periods. An overactive thyroid (hyperthyroidism) can cause nervousness, irritability, trouble sleeping, a fast or irregular heartbeat, shaky hands and weight loss. MedlinePlus lists both sets of symptoms. It adds that adults over 60 with an overactive thyroid may instead lose their appetite or withdraw from others. This can be mistaken for depression or dementia.
NICE advises doctors to consider thyroid tests for adults with depression or unexplained anxiety. It notes that in menopausal women, thyroid symptoms may be mistaken for the menopause. NICE advises offering tests to people with type 1 diabetes, another autoimmune disease or new atrial fibrillation. It advises against testing solely because someone has type 2 diabetes.
Most people with tiredness or low mood turn out to have a normal thyroid result. Our guide to tiredness covers the other common causes a GP considers.
Your report compares your numbers with the lab's reference range. The British Thyroid Foundation gives typical adult ranges. TSH is 0.4 to 4.0 mU/L, FT4 9.0 to 25.0 pmol/L and FT3 3.5 to 7.8 pmol/L. It stresses that these are only a guide and vary between labs. Use the range printed on your own report.
These patterns, from the British Thyroid Foundation, NICE and MedlinePlus, are a starting point, not a diagnosis. HSE laboratory advice notes that a value outside a reference interval is not always abnormal for that person. Equally, a value inside it does not exclude illness.
Sometimes FT4 is high or low while TSH is normal. HSE primary care guidance advises GPs to discuss this with a laboratory consultant or endocrinologist before treating.
"Subclinical" means TSH is outside the range but thyroid hormone levels are still inside it.
The HSE says this may mean you are at risk of developing a problem in the future, and your GP may suggest regular tests. The British Thyroid Foundation says it may gradually become a full underactive thyroid over many years. An antibody test helps show that risk.
Whether to treat is where guidance differs.
The evidence behind that panel was a 2018 meta-analysis of 21 randomised trials in 2,192 non-pregnant adults. Compared with placebo or no treatment, thyroid hormone brought TSH into the normal range. But it was not associated with better quality of life or fewer thyroid-related symptoms. The largest trial, TRUST, gave 737 adults aged 65 and over levothyroxine or placebo. After 1 year there was no difference in hypothyroid symptom scores or tiredness scores.
So a mildly raised TSH alongside tiredness does not mean treatment will fix the tiredness. That is a conversation to have with your GP.
NICE suggests considering specialist advice if 2 TSH readings, at least 3 months apart, are below 0.1 mU/L. This applies where there are also symptoms or signs of thyroid disease. Otherwise it suggests checking TSH every 6 months.
Repeat tests are normal and often deliberate:
The HSE guide also says screening healthy adults for thyroid problems is not warranted. For healthy people without symptoms, it suggests waiting at least 3 years after a normal result before retesting.
A positive antibody result does not always mean disease. The British Thyroid Foundation says positive antibodies can be found in people without thyroid disease. In subclinical disease, they suggest a higher chance of thyroid disease later, but not that treatment is needed yet.
Screening people without symptoms is not recommended either way in the US. In 2015, the US Preventive Services Task Force found the evidence insufficient to judge screening in non-pregnant adults without symptoms.
It is an ordinary blood test from a vein in your arm. MedlinePlus says no special preparation is usually needed, unless other tests taken at the same time require fasting. Tell the person taking your blood about every medicine and supplement you take.
If low mood or anxiety brought you here, see anxiety as a feeling or a disorder and what works for depression. If you are struggling or thinking of harming yourself, call Samaritans on 116 123 (Ireland and UK). In Ireland you can also text 50808. In an emergency, call 112 or 999.
There is no single number. The British Thyroid Foundation gives 0.4 to 4.0 mU/L as typical for adults, but ranges vary by lab, age and pregnancy. Ask your GP what your result means.
Many labs use a cascade. They measure FT4, and sometimes FT3, only if TSH is outside the range. NICE suggests this approach for most adults. If TSH is normal, the thyroid is very likely working normally.
Usually not. NICE suggests adding FT3 when TSH is low, mainly to assess an overactive thyroid.
MedlinePlus says no special preparation is usually needed. If other tests are being taken at the same time, follow the instructions for those. Mention any biotin supplement beforehand.
It can contribute. MedlinePlus lists depression among symptoms of an underactive thyroid, and nervousness among symptoms of an overactive one. NICE advises considering thyroid tests in depression or unexplained anxiety. A normal result points towards other causes.
This article is general information, not medical advice. It cannot tell you whether you need a test or what your result means for you. Talk to your GP or prescriber about your own situation, and do not change any medicine without their advice.
Reviewed by: Proco Team
Last updated: November 2026