Thyroid
Reverse T3
An inactive form of thyroid hormone; changes mainly with illness and fasting and says little about the thyroid.
Everything about this testWhat it measures
The thyroid mainly makes thyroxine (T4). In the tissues, the body converts T4 into active T3, or into reverse T3 (rT3), a form that has no hormone effect. This is how the body regulates how much active hormone is available. During illness, fasting and with some medicines, less T3 and relatively more rT3 is made. The test measures the amount of rT3 in the blood.
- Changed conversion of thyroid hormone in severe or long-lasting illness, fasting or medicine use
- Limited: no proven value for detecting or monitoring a thyroid disease
High
A raised rT3 is usually a normal response of the body to illness, surgery, fasting, a strict diet or malnutrition: the body then slows the conversion to active T3. Medicines such as amiodarone, corticosteroids and propranolol can also slow this conversion. With an overactive thyroid, rT3 can rise along with the other thyroid hormones. A raised rT3 with a normal TSH and free T4 is no proof of a thyroid disease or of a shortage of thyroid hormone in the tissues.
Low
A low rT3 rarely has meaning. It can fit an underactive thyroid, because there is then less T4 to convert, but TSH and free T4 show this much more reliably. With a normal TSH and free T4, a low rT3 says nothing about the thyroid.
If your result is abnormal
The GP assesses thyroid function with TSH and free T4. If these are normal, an abnormal rT3 is no reason for further thyroid follow-up or treatment.
Reliability
Reverse T3 changes mainly due to illness, fasting and medicines, and it has not been shown that the measurement adds anything to detecting or monitoring a thyroid disease, in people with or without symptoms. Thyroid function is determined with TSH and free T4 (Dutch GP guideline (NHG) on thyroid disorders), and even free T3 is only needed in specific cases. An abnormal rT3 can therefore not be interpreted on its own.
Limitations
- The value changes with any severe or long-lasting illness, with fasting and with diets, without the thyroid being diseased.
- Medicines such as amiodarone, corticosteroids and beta blockers (propranolol) affect the conversion of T4.
- Few laboratories measure rT3 and there are no generally agreed reference values; results from different laboratories are not easy to compare.
How common is this
An underactive thyroid affects about 3 to 4 in 100 adults in the Netherlands and an overactive thyroid about 1 in 100, more often women and older people. No Dutch figures are known for an abnormal rT3 as a stand-alone finding.
Who is this useful for
If you have symptoms that may fit a thyroid disorder, such as tiredness, TSH and free T4 are more useful: they show much more reliably how the thyroid is working. Reverse T3 can at most give additional information alongside those values. In people without symptoms, an abnormal rT3 says little, because the value also changes with illness, fasting and diets.
Downsides and risks
- A raised rT3 is often explained online as a hidden thyroid problem or as a reason for T3 medication; this explanation is not supported by evidence and can lead to unnecessary worry or unnecessary treatment.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results