The short answer
TSH is made by the pituitary gland to tell the thyroid how hard to work. When thyroid hormone is low, the pituitary shouts louder and TSH rises; when thyroid hormone is high, TSH falls. That inverse relationship is why a high TSH suggests an underactive thyroid and a low TSH suggests an overactive one. Because TSH moves early and sensitively, it is the usual screening test, with other tests added depending on what it shows.
What the pattern suggests
| Pattern | Usually points towards | Typical next step |
|---|---|---|
| High TSH, low free T4 | Underactive thyroid | Confirm and consider treatment |
| High TSH, normal free T4 | Mildly underactive — subclinical | Repeat after an interval; decide with symptoms in view |
| Low TSH, high free T4 | Overactive thyroid | Prompt further testing and often referral |
| Low TSH, normal free T4 | Mildly overactive, or another cause | Repeat and investigate |
| Normal TSH with symptoms | Thyroid is probably not the explanation | Look elsewhere: anaemia, sleep, mood, other causes |
That last row matters. Fatigue, weight change and low mood are common and mostly not thyroid — which is why a normal TSH should redirect the search rather than end it.
When thyroid testing is worth doing
- Symptoms that fit: fatigue, cold or heat intolerance, weight change, hair or skin change, palpitations, cycle change, constipation.
- A family history of thyroid disease or another autoimmune condition.
- A neck lump or goitre — which also needs examination.
- Monitoring an existing diagnosis or a dose change.
- Overlap with perimenopause symptoms — see perimenopause and menopause questions.
What makes a TSH unreliable
- Acute illness or hospitalisation, which shifts thyroid tests temporarily.
- Pregnancy, which has its own reference expectations.
- Biotin supplements, which interfere with several assays — stop them before testing if advised.
- Steroids, some psychiatric medications, amiodarone and others.
- Time of day and recent dose timing when already on thyroid medication.
Why repetition is normal
TSH fluctuates, and treating an isolated abnormal value can mean treating a blip for life. Repeating after an interval, with free T4 and sometimes antibodies added, is how a real picture is established. That is a considered approach, not a delay — and the details of the sequence are set out in what happens after an abnormal lab result.
What Presence can do
A virtual visit can take the history, order TSH and follow-on testing locally with the price quoted first, review results and manage stable thyroid replacement over time. A neck lump, a suspected overactive thyroid and anything needing examination or imaging go in person or to endocrinology. No visit guarantees a prescription.
Educational only. Do not adjust thyroid medication from this page — dose changes need testing at the right interval.