Thyroid
Beyond TSH: The Thyroid Tests That Actually Answer Your Questions
More thyroid tests are not always better. The right ones, in the right order, answer your question faster.
- Written by
- Farhat Ali Khan
- Published
- Reading time
- 3 minutes
A plain-English guide to TSH, free T4, free T3, thyroid antibodies and reverse T3: what each one tells you, when it is worth adding, and how to prepare so your results are reliable.
The short version
- TSH is the best single screening test for thyroid function.
- Free T4 confirms what TSH suggests and is essential if TSH is abnormal.
- Free T3 is mainly useful for an overactive thyroid.
- Anti-TPO antibodies show whether an autoimmune cause is behind the problem.
- Reverse T3 is rarely useful in routine care.
Sound like you? Our 2-minute health assessment shows which causes are worth checking first.
Search "thyroid test" and you will find packages listing a dozen thyroid markers. It is tempting to assume that more tests mean a more thorough answer. In practice, the right two or three tests, chosen for your situation, tell you more than a long list read without context.
TSH: the right first test
TSH is the signal from your pituitary gland telling the thyroid how hard to work. Because it responds to small changes in thyroid hormone, it is the most sensitive single screening test. A normal TSH on repeat testing makes a significant thyroid problem unlikely in most people.
When TSH is high, the thyroid is likely underactive. When it is low, it may be overactive. Either way, the next step is free T4.
Free T4: confirming the picture
T4 is the main hormone the thyroid produces. "Free" T4 is the portion that is available to your tissues, which is why it is preferred over total T4.
- High TSH, low free T4: an underactive thyroid (hypothyroidism).
- High TSH, normal free T4: subclinical hypothyroidism, which we explain in normal TSH but still exhausted?
- Low TSH, high free T4: an overactive thyroid (hyperthyroidism).
- Unusual combinations: for example a low TSH with a low free T4 can point to the pituitary gland, and needs a doctor's attention.
Free T3: useful in specific situations
T3 is the more active form of thyroid hormone, mostly made from T4 inside your tissues. Free T3 is mainly useful when an overactive thyroid is suspected, because some people overproduce T3 first. For diagnosing or monitoring an underactive thyroid, it adds little in most cases.
Thyroid antibodies: finding the cause
Anti-TPO antibodies show whether your immune system is attacking your thyroid, the hallmark of Hashimoto's thyroiditis, the most common cause of hypothyroidism. A positive result with a borderline TSH makes it more likely the thyroid will become underactive over time, which affects how closely you are monitored and whether treatment is considered.
For a suspected overactive thyroid, TSH receptor antibodies help confirm Graves' disease.
Reverse T3 and other extras
Reverse T3 is an inactive form of T3 that rises with illness, stress and calorie restriction. It features in many online "complete thyroid panels", but major thyroid guidelines do not recommend it for diagnosing or managing hypothyroidism, and a result rarely changes what a doctor should do. It is usually not worth paying for.
A thyroid ultrasound is useful if you or your doctor can feel a lump or swelling in the neck, not as a routine screening test.
How to prepare for a thyroid test
- Test in the morning, and at a similar time each time you retest.
- Pause biotin supplements for a few days beforehand, as biotin interferes with many thyroid assays.
- If you take thyroid medicine, ask your doctor whether to take it before or after the blood draw.
- Use the same lab where possible so results are comparable.
- Tell your doctor if you are pregnant or planning pregnancy, because target ranges are different.
Put the thyroid in context
Many symptoms blamed on the thyroid, such as fatigue, weight gain and brain fog, are also caused by insulin resistance, low iron or B12 and poor sleep. Testing those alongside your thyroid gives a more complete answer than adding more thyroid markers. Our 2-minute health assessment helps you decide what to test first.
Common questions
TSH. It is the most sensitive single test and the right first step for most people. Free T4 is added when TSH is abnormal or when the pituitary gland may be involved.
Fasting is not usually required for thyroid tests, but a morning sample at a consistent time gives the most comparable results. Ask your lab, especially if other tests are being done at the same time.
It means your immune system is targeting your thyroid, the hallmark of Hashimoto's thyroiditis. Many people with positive antibodies have normal thyroid function for years, but they are more likely to develop hypothyroidism, so regular monitoring makes sense.
Rarely. Reverse T3 changes with illness, stress and calorie restriction, and major thyroid guidelines do not recommend it for diagnosing or managing hypothyroidism.
It depends on your situation. People on thyroid medicine are usually tested every 6 to 12 months once stable. People with positive antibodies or borderline TSH are often retested yearly or sooner if symptoms change.
Sources
Written by Farhat Ali Khan, Co-founder, Opwel.
Published . Last updated .
General health information. It is not medical advice and it is not a substitute for a consultation about your own situation.
This article is general health information. It is not a diagnosis and it is not advice about your own health; speak to a doctor before changing your treatment, diet or supplements.
