In short
- TSH comes from the pituitary gland and measures the instruction to the thyroid, not the thyroid hormone itself.
- A high TSH fits an underactive thyroid, a low TSH fits an overactive thyroid.
- Free T4 belongs alongside it: only the combination makes a result usable.
- An abnormal TSH is almost always repeated before any conclusion follows.
What TSH actually measures
TSH stands for thyroid-stimulating hormone. It does not come from your thyroid, but from the pituitary gland, a small gland at the base of the brain. The pituitary works like a thermostat: it continuously senses how much thyroid hormone is circulating in your blood and adjusts using TSH.
If there is too little thyroid hormone, the pituitary turns the heating up and TSH rises. If there is too much, the pituitary eases off and TSH falls. So TSH does not measure your thyroid hormone itself, it measures the instruction being sent to the thyroid.
That is why the result works the opposite way to what most people expect. A high TSH fits a thyroid that is delivering too little, and a low TSH fits a thyroid that is delivering too much.
TSH is measured first because it is sensitive: the pituitary responds to small changes, often before you notice anything. That sensitivity is also the weakness of the test, because the value shifts with circumstances that have nothing to do with thyroid disease.
A high TSH: a sign of an underactive thyroid
If your TSH sits above the reference used by the laboratory, your pituitary is working harder to drive the thyroid. That fits an underactive thyroid, known medically as hypothyroidism.
Complaints that can go with it include:
- persistent fatigue
- feeling cold when others are comfortable
- weight gain without eating differently
- constipation, dry skin and dry hair
- slower thinking or low mood
- muscle aches and a slow heart rate
In women, periods can become heavier or more irregular. An autoimmune reaction against the thyroid, Hashimoto's disease, is the most common cause.
A low TSH: a sign of an overactive thyroid
If your TSH sits below the reference, that is often a sign that too much thyroid hormone is circulating and the pituitary is easing off. That fits an overactive thyroid, hyperthyroidism.
Recognisable complaints are:
- palpitations, inner restlessness and irritability
- poor sleep
- sweating and struggling with heat
- trembling hands
- losing weight while eating normally or even more
- looser stools
Here too an autoimmune cause is common, Graves' disease, but a temporarily inflamed thyroid or a nodule producing hormone on its own can give the same picture.
| Underactive thyroid | Overactive thyroid | |
|---|---|---|
| TSH | above the laboratory reference | below the laboratory reference |
| What the thyroid does | delivers too little hormone | delivers too much hormone |
| Pattern of complaints | slowed down: tired, cold, weight gain | revved up: palpitations, restlessness, weight loss |
| Common cause | Hashimoto's disease | Graves' disease |
Free T4, free T3 and anti-TPO: what do they add?
Free T4 is the thyroid hormone itself, in the form that is freely available to your tissues. Where TSH shows the instruction, free T4 shows what the thyroid is actually delivering. It is the combination that makes a result usable.
A raised TSH with a lowered free T4 points to a thyroid falling short, while a raised TSH with a normal free T4 is a milder picture that is often followed up rather than treated straight away. And if TSH and free T4 do not fit together logically, the problem may lie higher up the chain, in the pituitary itself. That is rare, but it is exactly why TSH is never assessed on its own.
Free T3 is the most active form of the hormone. It is not measured as standard, but it can refine the picture with a markedly abnormal TSH or with complaints that persist.
Anti-TPO are antibodies against thyroid tissue. If they are present, that points to an autoimmune process in the background and makes it clearer why the thyroid is out of balance. That is useful information for what comes next, because it says something about the chance that the function will change further. Antibodies are not a disease in themselves: they also occur in people whose thyroid is working perfectly well.
Have TSH and free T4 measured at Bloodworks
The thyroid test measures TSH together with free T4, all-in including the blood draw, doctor interpretation and a personal PDF report. No referral required.
Why one result is rarely the whole story
TSH follows a daily rhythm and is on average higher in the morning than later in the day, so the timing of the draw matters. The value can also shift temporarily through:
- an infection or a hospital admission
- pregnancy
- the contraceptive pill or other oestrogens
- medication such as corticosteroids, lithium, amiodarone or iodine-containing contrast fluid
High-dose biotin in supplements can also interfere with the laboratory measurement itself and produce a result that does not match your situation.
For those reasons an abnormal TSH is almost always repeated after a few weeks to months in practice, together with free T4. Only when the picture is stable and fits your complaints does a conclusion follow. Starting or adjusting treatment on the basis of a single measurement is not customary.
When a blood test is not the answer
If you feel a lump or swelling in the neck, have difficulty swallowing, a persistently hoarse voice or breathlessness, see your GP. That calls for a physical examination and usually an ultrasound, and a blood test cannot replace it.
With a wish to conceive or during pregnancy, thyroid values belong with your GP or midwife, because different considerations apply there. If you already use thyroid medication, monitoring belongs with the doctor who adjusts your dose.
And bear in mind that fatigue, weight gain and low mood far more often have another cause than the thyroid: anaemia, a deficiency of iron or vitamin B12, sleep apnoea, depression or simply prolonged overload. A normal thyroid result does not mean your complaints are imaginary, it means you need to look further.
Having your thyroid tested at Bloodworks
At Bloodworks you have your thyroid tested without a referral, at one of the more than 750 affiliated collection points across the Netherlands. The Thyroid function package measures TSH and free T4, the two values that form the basis together.
If you want to complete the picture, for example because you want to rule out an autoimmune cause or because complaints persist, there is an extended version that adds free T3 and anti-TPO. Every result is reviewed by a doctor and reaches you as a personal PDF report, typically within 2 working days, including whether a repeat test or a discussion with your GP is appropriate.
Frequently asked questions
Do I need to fast for a TSH test?
No, fasting is not needed for thyroid values. It is useful to have a follow-up measurement taken at roughly the same time of day as the first, because TSH follows a daily rhythm.
I take biotin. Does that matter?
High-dose biotin, often found in supplements for hair and nails, can interfere with thyroid measurements. Discuss with your doctor whether you can pause it for a few days before the draw, and in any case mention that you use it.
My TSH is abnormal but I have no complaints. What now?
That happens regularly. Repeating after a few weeks to months, together with free T4, is usually the logical step, so it becomes clear whether this is a temporary fluctuation or a lasting change.
Is anti-TPO always needed?
No. Anti-TPO is mainly useful when TSH and free T4 are already abnormal and the question is where that comes from, or when thyroid conditions run in your family. With normal thyroid function the test usually adds little.
Can I adjust my own dose based on this test?
No. If you use thyroid medication, the dose should be set by the doctor supervising that treatment. Our result can be a reason to contact that doctor sooner.