Is rT3 blocking your thyroid function?

In a normal functioning thyroid, the pituitary gland produces TSH - when a healthy thyroid gets this message, it produces two hormones - triiodothyronine (T3) and thyroxine (T4). T4 is converted to T3 on an as-needed basis to regulate metabolic functions.

Your liver also converts T4 to rT3 (reverse T3) which is inactive and actually blocks the action of T3. This is part of the body's normal process to keep your hormones in balance and get rid of unneeded T4. Normally about 40% of T4 is converted to T3 and 20% to rT3. However if the proportion of rT3 dominates then it can produce hypothyroid symptoms, even though your thyroid function test results may appear normal.


The balance of rT3 to T3 can be disrupted by a number of factors, most notably because of:

  • an inappropriate response to stress - which can result in too much cortisol, or too little cortisol in the case of Adrenal Fatigue
  • inadequate iron levels
  • other chronic illnesses

Understanding circulating rT3 levels with a reverse T3 test can provide important insights to understand the link between a thyroid disorder or adrenal hormone imbalance.

What we test

Reverse T3 Test

Reverse T3 (rT3) is virtually inactive having only 1% the activity of T3 and blocks the action of T3 acting as a metabolic break. Normal metabolism of T4 requires the appropriate balance of T3 to rT3. If the proportion of rT3 dominates then it will antagonise T3 thus producing hypothyroid symptoms despite sufficient circulating levels of T4 and T3.

This reverse T3 test measures:

Reverse T3 is an inactive form of the thyroid hormone T3. rT3 does not bind to thyroid hormone receptors and does not exert the same physiological effects. rT3 levels may be be increased during periods of stress, illness, or calorie restriction.

Test instructions

Download and print your pathology form from your i-screen dashboard.

Take your form to one of our affiliated collection centres to have your sample taken - no need for an appointment.

Results for this test available in 10-12 days and will be published in your online dashboard.

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