If your GP has ordered a thyroid blood test, or your results are full of letters like TSH and FT4, it helps to know what they mean. Here is each thyroid blood test explained in plain language, with questions to take back to your doctor.
Thyroid blood test explained: what do TSH, T4 and T3 measure?
Your thyroid makes the hormones T4 (thyroxine) and T3 (triiodothyronine), and the pituitary gland in your brain controls it with thyroid stimulating hormone (TSH), which rises when thyroid hormones drop and falls when they climb too high. Pathology Tests Explained — TSH
On your report you may see:
- TSH: the pituitary's signal, usually tested first. Healthdirect — Thyroid function tests
- Free T4 (FT4): the main thyroid hormone, in its unbound, active form. Pathology Tests Explained — Thyroid function tests
- Free T3 (FT3): less plentiful but more active, and mainly used to check for an overactive thyroid. Pathology Tests Explained — Thyroid function tests
- Thyroid antibodies, such as TPO or TSH receptor antibodies: checked when an autoimmune condition such as Hashimoto's thyroiditis or Graves' disease is possible. Pathology Tests Explained — Thyroid antibodies
A positive result does not always mean a thyroid problem: some healthy people have antibodies, and most of them never develop one. Pathology Tests Explained — Thyroid antibodies
Our simple introduction to the thyroid covers the gland itself.
Who orders thyroid tests, and why is TSH first?
Your GP may suggest thyroid tests if you have symptoms of an under- or overactive thyroid, unexplained changes to your periods or difficulty getting pregnant, or to monitor thyroid medicine. Healthdirect — Thyroid function tests
They usually start with TSH, the only thyroid test Medicare covers for people without a history of thyroid problems; if it is high or low, the lab often adds free T4 and sometimes free T3. Pathology Tests Explained — Thyroid function tests
A high TSH may point to an underactive thyroid (hypothyroidism) and a low TSH to an overactive one (hyperthyroidism), but the dose of thyroid medicine or a pituitary problem can also shift it. Healthdirect — Thyroid function tests
TSH can also rise during an illness, so your doctor may recheck it after you recover. Pathology Tests Explained — TSH
Why do reference ranges differ between labs?
Your result is compared with a reference interval, the range expected in healthy people, and the TSH range varies between labs because they use different instruments and methods. Pathology Tests Explained — TSH
So read each result against the range on the same report, and remember that a high (H) or low (L) flag does not necessarily mean something is wrong. Pathology Tests Explained — TSH
Context matters too: in early pregnancy TSH can be lower, because the pregnancy hormone hCG acts a little like TSH. Pathology Tests Explained — TSH
How can you prepare for a thyroid blood test?
- Tell your GP about every medicine and supplement you take, since many can affect thyroid results, and ask whether you need to fast. Pathology Tests Explained — Thyroid function tests
- Mention biotin, common in hair and nail supplements: Pathology Tests Explained advises not taking it for two days before a blood test, so check with your GP. Pathology Tests Explained — Thyroid function tests
- If you take thyroid hormone replacement, keep taking it as prescribed, even when your levels are back in range and you feel better. Healthdirect — Hypothyroidism
- After a dose change, TSH is usually rechecked after at least one to two months, once the new dose has taken full effect. Pathology Tests Explained — Thyroid function tests
What can you ask your GP about your results?
Bring copies of earlier results and a few notes on how you have been feeling. You could ask:
- Why is this test being done, and what should it tell us?
- Which tests were done: TSH only, or free T4, free T3 or antibodies too?
- Where does my result sit in this lab's range, and what does that mean for me?
- Could an illness, pregnancy, a medicine or a supplement have affected it?
- Do we need to repeat it, and when?
Our post on preparing for a GP conversation about thyroid symptoms has more ideas.
When to see your GP
See your GP if you notice:
- possible signs of an underactive thyroid, such as tiredness, feeling the cold, constipation, dry skin or unexplained weight gain Healthdirect — Hypothyroidism
- possible signs of an overactive thyroid, such as feeling anxious, feeling unusually hot, heart palpitations or losing weight without trying Healthdirect — Hyperthyroidism
- a swelling at the front of your neck Healthdirect — Hyperthyroidism
- new or worsening symptoms while you are taking thyroid medicine Healthdirect — Hypothyroidism
These symptoms overlap with many other conditions, so also mention any family history of thyroid or autoimmune conditions. Healthdirect — Hypothyroidism
If you have a thyroid condition and are planning a pregnancy, are pregnant or are breastfeeding, talk to your doctor, as medicine doses may need to change. Healthdirect — Hyperthyroidism
What can we talk about at Melbourne Wellness?
Thyroid tests, diagnosis and thyroid medicine stay with your GP or specialist. At an initial naturopathy consultation we can go through your health history and the results you bring, talk about diet, sleep and stress, and help you prepare questions for your GP.
We do not change or replace thyroid medicine, and if anything suggests you need a medical review, we will say so.
