Learn about high reverse T3 levels and if they should be part to a thyroid test for your next checkup.
Reverse T3 (rT3) is the third most common iodine containing thyroid hormone.
When an atom of iodine is removed from T4 (which has 4 iodine atoms), you get T3 or rT3 depending on which position sheds the iodine.
The sketch below shows the two possible positions of iodine release (a process called deionization).
T3 and rT3 have very different reactivities. The more abundant form, T3 is very active; however, reverse T3 is fairly inactive and is only observed during certain health conditions.
One of the common conditions with high reverse T3 is non-thyroidal illness syndrome (NTIS). NTIS is a rare condition due to serious illnesses or starvation when no carbohydrates are available to the body.
NTIS diagnosis also involves very high TSH levels. Therefore, a reverse T3 test can be useful in such complicated situations, especially when medication can interfere in diagnosis of hypothyroidism.
High reverse T3 levels are also common in newborns but quickly drop after delivery. That's mainly due to the enzyme responsible for converting T4 into rT3.
Reverse T3 hormone levels increase with age in adults. In contrast, T3 tends to decline with age.
People with elevated reverse T3 have high mortality rate (hazard ratio of 1.23; equal rates means ratio of 1). That's because high levels are often found in people with serious illnesses or starvation.
High T3 to reverse T3 ratio has been linked to higher insulin resistance, something a TSH test can not capture. And women tend to have higher ratio than men.
Elevated reverse T3 levels are also involved in breast cancer and in tumors of brain and spine.
Many drugs can affect thyroid function because they interfere with the enzymes involved in T4 to rT3 conversion.
A reverse T3 test is sometimes used in diagnosis of hypothyroidism in such scenarios of drug interference.
Amiodarone is a drug taken for heart arrhythmia that has a high iodine content. Amiodarone is well known to cause elevated reverse T3 hormone levels, as it can prevent clearing of rT3.
Such iodine-rich Amiodarone treatment also raises TSH & T4 but lowers T3 levels.
A reverse T3 test during amiodarone treatment can help in diagnosis of hypothyroid patients, independent of the rise in TSH levels.
Dexamethasone causes elevated reverse T3 levels. It increases the T4 conversion to rT3, instead of the normal T4 to T3 conversion.
Isoflavones in soy and beans mimic the behavior of estrogen. They are also known to result in high reverse T3 hormone, albeit for short bursts. They can cause short-term increase in TSH levels.
Clomipramine is an antidepressant that can cause short-term low rT3 levels.
Glucocorticoid steroid hormones used as anti-inflammatory medicines can also result in short term spike in reverse T3.
Some iodine containing solutions given during X-ray imaging to enhance the contrast, e.g., Ipodate, can also cause short term elevated reverse T3 levels.
Anti-thyroid drugs, such as PTU (propylthiouracil) can also cause high reverse T3 levels.
The synthetic T4 replacement, thyroxine, can also cause high rT3.
Reverse T3 testing is not common but can aid in diagnosis of certain thyroid problems.
Here are a few unique conditions where rT3 testing can be useful:
Genetic disorders of deionization enzyme that can prevent normal growth, cause deafness, weak muscles and infertility; such genetic disorders are confirmed with high rT3 and T4, relatively lower T3 levels.
Mutations in transporting thyroid hormones can prevent metabolism of T3, T4; these show up as low rT3, somewhat low T4 but high T3 & above normal TSH.
High reverse T3, along with T4 levels, are also observed in people with mutations in the proteins that bind with thyroid hormones.
Elevated reverse T3, along with T3 and rT3 ratio, can be used to learn about problems with receptors that bind thyroid hormones. Receptors in the pituitary gland (THR-beta) show high rT3, T3, and T4 levels; those in other organs, such as heart, lungs, nerves, bones, intestines (THR-alpha) show low rT3 and T4 but high T3 and TSH.
Reverse T3 is a less known thyroid hormone that's has slightly different structure than regular T3. This inactive form is found in certain health conditions and can serve as a marker to aid diagnosis of hypothyroidism and other thyroid problems.
Talk to your doctor if you need to consider reverse T3 test. Or if your symptoms are not easily explained, consider ordering a test to learn if they might be due to high reverse T3 (rT3) levels.
Order an A Comprehensive Thyroid Test.
All About Thyroid - an in depth summary.
Normal TSH Levels: What's Normal and Why? - a detailed look at thyroid stimulation hormone.
Five Common Thyroid Problems To Watch Out For - a review of most common problems.
The Difference Between Hypothyroidism and Hyperthyroidism - a quick look at key differences.
Thyroid and Pregnancy - about the critical role thyroid plays during pregnancy.
Thyroid and Iodine - thyroid problems depend on lack or excess of iodine.
High TSH and normal T4 levels - check these markers for early signs of thyroid problems.
