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:
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.
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.
Reverse triiodothyronine is an isomer of triiodothyronine (T3) which blocks its action.
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