Reverse T3 testing: what the evidence says

Reverse T3 is a real hormone that can be measured, but the main clinical sources do not treat it as a useful way to diagnose or manage an underactive or overactive thyroid. The British Thyroid Foundation lists the usual blood tests for thyroid function as TSH, T4 and sometimes T3, and notes that in most cases TSH is measured first because if it is normal the thyroid is very likely to be working normally.

If you have been offered a reverse T3 result as part of a private panel, or you are reading a result that has already come back, the points below should help you decide how much weight to put on it and what to ask your own doctor.

The thyroid gland makes mostly thyroxine (T4). The British Thyroid Foundation explains that T4 is converted into tri-iodothyronine (T3), and that it is T3 that is biologically active and regulates metabolism. The American Thyroid Association adds that T4 can also be broken down into reverse T3, which is inactive, and that this mostly happens inside cells in other tissues rather than in the thyroid itself.

The same source notes that T3 and rT3 move in opposite directions: when T3 is higher, rT3 is lower, and when T3 is lower, rT3 is higher. Because rT3 is inactive and mirrors T3, most clinicians do not measure it when assessing whether someone has hypothyroidism or hyperthyroidism.

In practical terms, then, a reverse T3 result is a measure of a by-product, not of the hormone that does the work in your cells. This is one reason it is hard to turn a single number into a treatment decision.

Several independent sources point the same way. The ADLM (Association for Diagnostics and Laboratory Medicine) guide, last reviewed in March 2023, states that rT3 has limited applications, should not be used for thyroid function evaluation, is not a diagnostic or confirmatory test, and that TSH is the recommended main screening test for thyroid dysfunction in most clinical circumstances. It lists TSH and free T4 as the most commonly measured hormones for evaluating thyroid function.

The American Thyroid Association public summary of a laboratory-ordering study reached a similar conclusion. It reported that most rT3 tests were ordered by a small proportion of clinicians, and that the review of published literature did not support routine measurement of rT3 in hypothyroidism or hyperthyroidism. It also noted that this matters for patients who have to pay for their own tests.

In the UK, the NHS describes diagnosis of an underactive thyroid as a conversation about symptoms and family history, a possible neck examination, and blood tests for TSH and T4, with referral to a specialist for further tests if needed. A Scottish Parliament committee report on thyroid treatment, which considered a petition asking for fuller thyroid testing, records that the clinical effectiveness of the rT3 test is contested.

None of this means the test is meaningless. The ADLM guide says rT3 may help in investigating rare genetic syndromes, including resistance to thyroid hormone due to a defective thyroid receptor alpha and monocarboxylate transporter 8 (MCT8) deficiency. These are specialist diagnoses, and they are not what most people mean when they ask about reverse T3.

The guide also explains that rT3 rises when the body is under severe stress, in sick euthyroid syndrome, and with medicines such as amiodarone that inhibit its metabolism. It adds that it is generally unnecessary to use rT3 to diagnose sick euthyroid syndrome, and that it is not clear whether the results point to any specific treatment plan.

• Rare inherited thyroid hormone transport or receptor problems, investigated by specialists.

• Interpreting odd results during serious illness, where the picture is usually clear from the clinical context.

• Understanding the effect of medicines such as amiodarone on thyroid hormone metabolism.

This table summarises how the tests are described in the sources used for this page.

Symptoms such as tiredness, weight gain, low mood and dry skin are listed by the NHS as common symptoms of an underactive thyroid, but it also stresses that they are common and can be caused by other conditions. That is why a GP will look at the whole picture rather than a single hormone.

The British Thyroid Foundation also notes that current guidelines do not advise thyroid hormone replacement when thyroid hormone levels are within normal limits, and that blood tests are interpreted together with symptoms and how you feel. If your TSH and thyroxine are normal but you still feel unwell, sensible next questions are about other causes of your symptoms, such as iron or vitamin deficiencies, sleep, mood, other hormones and medicines, rather than a bigger thyroid panel.

Lab Tests Online UK warns that biotin (vitamin B7) supplements can cause test interference and false results in a thyroglobulin test, so telling the person arranging your blood test about every supplement you take is worthwhile whichever test you are having.

On the NHS, a reverse T3 test is not part of the usual initial investigation of thyroid symptoms. The Scottish Parliament committee report records that laboratories sometimes declined tests that clinicians had ordered, including rT3, and that practice varies, so a request that is declined is not unusual.

Private providers do offer reverse T3 within extended thyroid panels. The American Thyroid Association summary noted that a small group of providers ordered most rT3 tests, and that people paying for tests themselves may wish to weigh this. A reasonable approach is to ask any provider what decision the result would change, and to share any private result with your GP or specialist rather than acting on it alone.