TSH / THYROID
TSH and thyroid test results explained
TSH (thyroid-stimulating hormone) is made by the pituitary gland to tell your thyroid how much hormone to produce. Because it works like a thermostat, a high TSH usually goes with an underactive thyroid and a low TSH with an overactive one — which is the opposite of what most people expect.
Why it’s confusing
People read “high” as “overactive”. With TSH it tends to be the other way round: when thyroid hormone levels are low, the pituitary makes more TSH to push the thyroid harder; when they are high, it makes little or none.
Thyroid results also come as a set — TSH, free T4 and sometimes free T3 — and the pattern across them is what doctors read.
How to read it
Start with TSH, then look at free T4 (FT4). Some labs test TSH first and add FT4 automatically if TSH is abnormal.
Lab Tests Online UK sets out the usual patterns: high TSH with low T4 suggests hypothyroidism; high TSH with normal T4 suggests mild (subclinical) hypothyroidism; low TSH with high T4 or T3 suggests hyperthyroidism. For people taking thyroid replacement, a low TSH may suggest the dose is too high.
TSH can be outside its range when the thyroid is healthy. MedlinePlus notes that some serious illnesses, pregnancy and age over 80 can all affect TSH.
What it does not mean
- That high TSH means an overactive thyroid. It usually points the other way.
- That a TSH result tells you the cause. Other tests, such as thyroid antibodies, look for that.
- That you should change a thyroid medicine dose because of a result. Talk to your doctor first.
Where you’ll see it
- Thyroid function test (TFT) results.
- Routine blood tests and monitoring for people taking levothyroxine.
Questions worth asking
Ask your doctor
- What do my TSH and T4 results show together?
- Do I need any more thyroid tests?
- Could an illness or a medicine have affected this?
Got thyroid results you’d like explained? Attach the report or paste the text, and Clariti will walk through each line.
Common questions
- Does high TSH mean an overactive thyroid?
- Usually the opposite. A high TSH generally goes with an underactive thyroid, because the pituitary is pushing a slow thyroid harder.
- What is free T4?
- The free, active form of thyroxine, the main hormone the thyroid makes. Most UK labs measure the free form.
- Can TSH be abnormal with a healthy thyroid?
- Yes. MedlinePlus notes that serious illness, pregnancy and being over 80 can all affect TSH.
Sources
Reading a scan report rather than a blood test or letter? Radily’s radiology report phrase guides explain the wording radiologists use.