Thyroid Stimulating Hormone (TSH)
TSH is the signal your brain sends to tell your thyroid how hard to work. A TSH blood test is the usual first check of thyroid health — a normal result largely rules out a thyroid problem, while a result outside the range tells your GP where to look next. It is one of the most common tests in Australia, and most issues it picks up are common and manageable.
Standard CareIn an older Australian population, around 14% had thyroid disease — about 10% already identified with thyroid disease, plus 3.6% with an abnormal TSH and not yet picked up — a reminder that thyroid problems are common and often go unnoticed.
What is thyroid stimulating hormone (tsh)?
Thyroid-stimulating hormone (TSH) is produced by the pituitary gland — a small structure at the base of your brain that acts as a master controller for several hormones. TSH's job is to signal the thyroid (the butterfly-shaped gland at the front of your neck) to produce the thyroid hormones that regulate your metabolism, energy levels and body temperature. The system works as a feedback loop: when thyroid hormone falls, the pituitary releases more TSH to push the thyroid harder; when thyroid hormone rises, the pituitary dials TSH back down. Because TSH responds to even small changes in thyroid hormone, it is the most sensitive early signal of whether the thyroid is keeping up — which is why it is usually the first test a GP requests when a thyroid problem is suspected.
Why is it tested?
A GP usually starts with TSH because a normal result largely rules out a significant thyroid hormone excess or deficiency in one go — no other single test does that as reliably. It is commonly requested for symptoms such as ongoing tiredness, unexpected weight changes, feeling cold when others are not, hair thinning, or a faster or slower heart rate. It is also used to monitor people already taking thyroid medication. If TSH sits outside the range, your GP will usually measure free T4 (the main thyroid hormone circulating in your blood) and sometimes free T3 (the more active form) to clarify the picture.
Reference range (Australia)
Lab- and assay-dependent — the range printed on your own report is the one to use. TSH is interpreted alongside free T4 (and free T3 if indicated), and pregnancy and age can shift the appropriate range.
Reference intervals vary between laboratories. Always use the range printed on your own report.
What your result means
A TSH below the range may suggest the thyroid is overactive — doctors call this hyperthyroidism — or it can relate to thyroid medication, early pregnancy, or occasionally a pituitary issue, depending on what thyroid hormone levels show. Your GP will weigh your symptoms and usually arrange a free T4 result before drawing conclusions. A chat with your GP will sort out which explanation fits your situation.
An in-range TSH suggests the pituitary and thyroid are communicating well and thyroid hormone levels are likely adequate. Interpretation still depends on your symptoms, age and any thyroid medication, so your GP reads it in the context of your full picture.
A TSH above the range may suggest the thyroid is underactive — doctors call this hypothyroidism. The pituitary detects that thyroid hormone is low and responds by releasing more TSH to try to push the thyroid harder. TSH alone does not confirm the full picture; your GP will consider your symptoms, age, and usually a free T4 result before drawing conclusions. This is common and manageable, and there is no need to worry in isolation — worth raising with your GP when you can.
What can affect your result
Several things can shift a TSH result. Severe illness or starvation can disturb the normal feedback loop — a pattern sometimes called sick euthyroid (where the thyroid itself is fine but the readings look off) — so testing is often deferred until you are well. High-dose biotin supplements (commonly found in hair, skin and nail products) can interfere with some thyroid assays and produce falsely high or falsely low results; mention them to your GP before your test. Thyroid medication also changes TSH directly: levothyroxine (the most common thyroid hormone replacement tablet) raises TSH when underdosed and lowers it when overdosed, and inconsistent timing of doses can affect results. Some medicines — including amiodarone (a heart rhythm drug) — can alter thyroid function. Pregnancy and age both shift the expected range. A single result is less informative than a trend, so your GP may repeat the test before acting.
When to act
If your TSH is outside the range, it is worth a conversation with your GP. They will assess your symptoms, review your medications and supplements, and may arrange a free T4 result to clarify the picture before any decision is made. As a safety note, do not start, stop or change thyroid medication on your own; thyroid dosing is individual and your GP will arrange appropriate follow-up. This page is general information and is not a substitute for advice from your GP.
Frequently asked questions
Test your thyroid stimulating hormone (tsh)
NATA-accredited testing. Results in 2–5 days. No GP referral required.
View TSH