TSH, FT3, FT4: Which Thyroid Tests Do You Actually Need?

Fatigue, sleepiness, trouble losing weight, hair loss, feeling cold. You type the symptoms into a search engine and one suggestion shows up almost immediately: "get your thyroid checked."

But checked how, exactly?

TSH alone? TSH together with FT3 and FT4? Or anti-TPO, anti-Tg and a thyroid ultrasound right away?

More tests do not always mean better diagnostics. With the thyroid it matters especially that the tests are matched to a specific clinical situation, because each parameter answers a completely different question.

In most adults, testing can indeed start with TSH. There are situations, however, where that is not enough.

TSH is not a thyroid hormone

This is one of the first misunderstandings worth clearing up.

TSH, or thyrotropin, is produced in the pituitary gland, not in the thyroid. Its job is to stimulate the thyroid to produce hormones.

Think of it as a thermostat.

If the body "sees" too little thyroid hormone, the pituitary raises TSH production to stimulate the thyroid harder. If there is too much hormone, TSH usually drops.

That is why primary hypothyroidism is characterised by elevated TSH, while in hyperthyroidism we most often see it suppressed.

It is exactly this sensitivity that makes TSH the best starting test for thyroid function in most adults.

What does FT4 show?

The thyroid mainly produces thyroxine, or T4.

Most T4 circulates in the blood bound to proteins. The FT4 test measures the free fraction, the part not bound to transport proteins.

FT4 is especially useful when the TSH result is abnormal.

High TSH with low FT4 corresponds to overt primary hypothyroidism.

If TSH is elevated but FT4 stays within the reference range, that may correspond to subclinical hypothyroidism.

Low TSH with elevated FT4, on the other hand, may point to hyperthyroidism.

And what about FT3?

FT3, or free triiodothyronine, often ends up in the "thyroid panels" offered by laboratories. That does not mean it needs to be measured every time the thyroid is checked.

T3 is a biologically very active hormone. A large share of it is produced outside the thyroid, by converting T4 into T3 in various tissues.

FT3 is particularly useful in diagnosing hyperthyroidism.

In some people TSH is already clearly suppressed, FT4 is still normal, and it is T3 that is elevated. This picture can occur in early hyperthyroidism, among other situations.

When hypothyroidism is suspected, though, FT3 matters far less. T3 levels can remain normal even in clear hypothyroidism, so a normal FT3 does not rule out disease. The American Thyroid Association notes that measuring T3 is rarely helpful in diagnosing hypothyroidism.

So there is no need to run the "full set" of TSH, FT3 and FT4 at every check-up.

Which test should you start with?

In most adults without suspected pituitary disease, current guidance suggests starting with TSH.

Simplified, the pathway looks like this:

This diagnostic model appears both in the NICE guidance and in the thyroid disease prevention report prepared by Poland's AOTMiT. If you are wondering what else makes sense in routine screening, our list of preventive tests worth doing once a year will help.

Why is TSH alone sometimes not enough?

The most important exception involves disorders of the pituitary or hypothalamus.

In typical primary hypothyroidism the problem sits in the thyroid itself, so a drop in FT4 drives TSH up.

If the problem sits higher up, at the level of the pituitary, that mechanism may not work properly.

You can then see low FT4 with a TSH that is not elevated.

Looking at TSH alone, such a case could be missed.

That is why, when secondary or central hypothyroidism is suspected, TSH has to be interpreted together with FT4.

When is it worth testing the thyroid at all?

The symptoms of hypo- and hyperthyroidism are often non-specific. Fatigue or worsening concentration can stem from dozens of other causes, which is why it is worth first checking when chronic fatigue is a lifestyle issue and when it calls for diagnostics.

A single non-specific symptom therefore does not automatically mean thyroid disease.

Testing is particularly worth considering when several symptoms appear together, or when there are additional risk factors: unexplained weight change, persistent feeling of cold or heat, palpitations, hand tremor, menstrual irregularities, marked weakness, constipation, excessive sleepiness, an enlarged thyroid, new atrial fibrillation, other autoimmune conditions, or abnormal previous results. Testing is also worth considering individually with fertility problems, when planning a pregnancy, and during pregnancy.

The thyroid is also one of the directions to consider in hair loss, where the test panel is worth choosing deliberately.

None of this means that every healthy person without symptoms needs to run an extended "thyroid panel" regularly.

Anti-TPO: when is it worth measuring?

TSH and FT4 speak mainly to thyroid function.

Antibodies help answer a different question:

why is the thyroid not working properly?

Anti-TPO, antibodies against thyroid peroxidase, are the most commonly used marker of thyroid autoimmunity.

Measuring them is worth considering especially when TSH is elevated and we want to assess whether the cause could be autoimmune thyroiditis, that is Hashimoto's disease.

Importantly, antibodies are not there to assess the thyroid's current "capacity".

You can have high anti-TPO alongside perfectly normal TSH and FT4.

The reverse is also possible: hypothyroidism does not always stem from Hashimoto's.

Current NICE guidance recommends considering anti-TPO measurement in adults with elevated TSH. At the same time, regularly repeating anti-TPO to monitor treatment is not recommended.

Do you need anti-Tg?

Anti-Tg are antibodies against thyroglobulin.

They can appear in autoimmune thyroid disease, but in typical Hashimoto's diagnostics they matter less than anti-TPO.

So there is no need to run anti-Tg with every thyroid check.

According to European Thyroid Association guidance, measuring them can be considered when there is clinical or ultrasound suspicion of chronic autoimmune thyroiditis but anti-TPO remains negative.

And when are TRAb measured?

These are antibodies directed against the TSH receptor.

Measuring them matters particularly when Graves' disease is suspected, one of the most common causes of hyperthyroidism.

If TSH is suppressed, FT4 or FT3 elevated, and the clinical picture suggests Graves' disease, positive TRAb can confirm the diagnosis without the need for some additional tests.

TRAb also matter in certain situations during pregnancy, because the antibodies can cross the placenta.

Is a "preventive" thyroid ultrasound worth it?

Ultrasound and hormone tests answer two different questions.

TSH, FT4 and FT3 speak to thyroid function.

Ultrasound shows its structure.

You can have a perfectly normal TSH and a thyroid nodule at the same time. You can also have hypothyroidism without structural changes that require an ultrasound assessment.

So ultrasound does not replace hormone testing, and hormone testing does not replace ultrasound.

Ultrasound matters particularly with a palpable nodule, an enlarged thyroid, gland asymmetry or other clinical indications. Current recommendations do not, however, support imaging everyone simply because their TSH is abnormal.

Can Hashimoto's be diagnosed from high antibodies alone?

Not quite.

Positive antibodies indicate thyroid autoimmunity, but they say nothing yet about whether the thyroid is functioning normally.

Someone with positive anti-TPO can have normal TSH and FT4 for many years.

In that case we do not treat "an antibody result". What we monitor is thyroid function.

The American Thyroid Association stresses that repeating antibody levels regularly is not necessary. Monitoring thyroid function relies mainly on TSH and, where appropriate, FT4.

What about reverse T3?

Reverse T3, or rT3, shows up more and more often in online "extended hormone panels".

In routine diagnostics of hypothyroidism in an ambulatory patient, however, measuring it has no proven clinical value.

The American Thyroid Association notes that rT3 does not help diagnose hypothyroidism in healthy, non-hospitalised people.

So instead of ordering ever larger panels, it is better to pick a few parameters that answer the specific problem.

When is it not worth testing the thyroid?

There is one more important exception: acute illness.

Severe infection, hospitalisation, surgery or another significant illness can temporarily change TSH, FT4 and especially T3, even though the thyroid itself is not diseased.

This is known as non-thyroidal illness syndrome.

That is why NICE advises against routine thyroid testing during acute illness, unless a thyroid disorder is suspected to be the cause of the current state.

How to prepare for TSH, FT3 and FT4 testing

For a single measurement, what matters most is interpreting the result correctly in the context of the laboratory and the clinical situation.

For follow-up testing, though, it is worth keeping the sampling conditions similar.

TSH shows diurnal variation, so when comparing consecutive results it helps to test at a similar time of day. Current laboratory recommendations suggest drawing blood in the morning and, for maximum standardisation, fasting.

Pay particular attention to biotin, often found in "hair and nails" supplements.

Biotin can interfere with some laboratory methods and cause, among other things, artificially low TSH and falsely elevated FT4 or T3. The American Thyroid Association recommends stopping biotin at least 2 days before testing, and with higher doses a longer break is sometimes advised. The 2026 European Thyroid Association guideline likewise stresses the need to account for drug and supplement interference when results look atypical.

What if I take levothyroxine?

Here, reproducible testing conditions matter especially.

FT4 rises temporarily after taking levothyroxine. So for a follow-up that includes FT4, it is best to draw blood before the morning dose and take the medication afterwards.

Current laboratory recommendations point to exactly this pattern as the most comparable between consecutive tests.

That does not mean a single accidental tablet before the test completely invalidates the result. It is worth telling your doctor about it, though.

Levothyroxine itself should be taken as consistently as possible. The 2025 European Thyroid Association guidance recommends leaving at least 30 minutes between the dose and food, and ideally taking it about an hour before breakfast or in the evening, at least 3 hours after the last meal.

How often should TSH be checked during treatment?

TSH does not respond immediately to a change in levothyroxine dose.

Testing a few days after a treatment change therefore usually adds little.

NICE indicates that when reassessing thyroid function, tests should generally not be repeated sooner than about 6 weeks, unless there are specific clinical indications.

In adults on levothyroxine, TSH can be checked roughly every 3 months until results stabilise, and then usually once a year. In people with persistent symptoms despite treatment, FT4 can be added. The same logic applies to other parameters: it helps to know how often to repeat lab tests when the results are normal.

The thyroid in pregnancy plays by slightly different rules

Pregnancy changes the physiology of the thyroid axis.

TSH and thyroid hormone levels shift with each trimester, so results should not automatically be interpreted by the same rules as outside pregnancy.

Appropriate reference ranges and previous thyroid disease both matter.

Women already taking levothyroxine before pregnancy need particularly close monitoring, because hormone requirements can increase.

Fertility problems and assisted reproduction are a separate topic. The 2021 European Thyroid Association guideline recommends assessing thyroid function in women undergoing ART, and in patients treated for hypothyroidism the aim before starting treatment is an appropriately controlled TSH.

Summary

Not everyone needs TSH, FT3, FT4, anti-TPO, anti-Tg and an ultrasound all at once.

In most cases, testing can proceed in stages.

In an adult without suspected pituitary disease, we usually start with TSH.

If TSH is elevated, the most important next test is FT4.

If TSH is low, it is worth assessing FT4 and FT3.

Antibodies help identify the cause of the disturbance, and ultrasound assesses the structure of the gland. None of these tests should be interpreted in isolation from the others.

So the key question is not:

"How do I order the most extensive thyroid panel?"

But:

"Which test do I need to answer this specific problem?"

Frequently asked questions

Is TSH enough for a preventive thyroid check?

In most adults without pituitary disease and without previously diagnosed thyroid disease, TSH is the best starting test. If the result is abnormal, the panel can be widened accordingly.

Do TSH, FT3 and FT4 always have to be tested together?

No. In diagnosing primary hypothyroidism, FT3 is usually not needed. If TSH is elevated, the key additional parameter is FT4. FT3 matters more when hyperthyroidism is suspected.

Does a normal TSH rule out every thyroid disease?

No. In most people a normal TSH argues for normal thyroid function, but there are exceptions. One is central hypothyroidism linked to pituitary disease, where FT4 has to be assessed. A normal TSH also does not rule out thyroid nodules.

Does high anti-TPO mean hypothyroidism?

No. High anti-TPO indicates autoimmunity, but it does not define current thyroid function. That is what TSH and FT4 describe.

Is it worth monitoring anti-TPO regularly?

Usually not. Once Hashimoto's has been diagnosed, there is no need to track antibody levels regularly to judge how well treatment is working.

Should FT3 sit in the upper part of the range?

There is no universal recommendation to "optimise" FT3 to a particular spot in the reference range in a healthy person. The result always has to be interpreted alongside TSH, FT4, symptoms, treatment and the clinical situation.

Does a TSH of 4 mIU/L mean hypothyroidism?

That cannot be determined from a single number. Reference ranges depend on the laboratory, age, pregnancy and the clinical situation. With an elevated TSH, FT4 assessment is needed, and sometimes a repeat measurement.

Does trouble losing weight mean I need my thyroid tested?

Hypothyroidism can affect body weight and energy expenditure, so with matching symptoms it is worth ruling out. A stalled scale on its own, however, is not sufficient evidence of thyroid disease. We describe the scale of that effect in our piece on hypothyroidism and weight gain.

Can an ultrasound tell me whether I have hypothyroidism?

No. Ultrasound shows the structure of the thyroid, not its function. Hypothyroidism is diagnosed primarily from laboratory tests.

References

1. Jonklaas J, et al. Thyroid Stimulating Hormone and Thyroid Hormones (Triiodothyronine and Thyroxine): An American Thyroid Association-Commissioned Review of Current Clinical and Laboratory Status. Thyroid. 2023.
2. National Institute for Health and Care Excellence. Thyroid disease: assessment and management. NICE Guideline NG145, updated 2023.
3. Agency for Health Technology Assessment and Tariff System (AOTMiT). Prevention of thyroid disease. Report on recommended medical technologies. 2024.
4. Durante C, et al. 2023 European Thyroid Association Clinical Practice Guidelines for thyroid nodule management. European Thyroid Journal. 2023.
5. Centanni M, et al. ETA guidelines for the use of levothyroxine sodium preparations in monotherapy to optimize the treatment of hypothyroidism. European Thyroid Journal. 2025.
6. Poppe K, et al. 2021 European Thyroid Association Guideline on Thyroid Disorders prior to and during Assisted Reproduction. European Thyroid Journal. 2021;9(6):281–295.
7. National recommendations of the Croatian society of medical biochemistry and laboratory medicine: Thyroid function tests from the laboratory point of view. 2025.
8. European Thyroid Association. 2026 ETA guideline on interference in immunoassay measurements used in assessment of thyroid function. European Thyroid Journal. 2026.
9. Ylli D, et al. Biotin interference in assays for thyroid hormones, thyrotropin and thyroglobulin. Thyroid. 2021;31:1160–1170.

This article is educational and does not replace a medical consultation. The scope of thyroid testing and the interpretation of results are always worth discussing with your doctor.