Reverse T3 (rT3): What Your Result Means

What is Reverse T3 (rT3)?

When the body converts T4 into active T3, a small share is instead converted into reverse T3 — an inactive form. Under stress, illness, very low calorie intake, or thyroid dysfunction, the balance shifts toward reverse T3. Because it competes for the same receptors, high rT3 can blunt thyroid hormone action, which is why thyroid enthusiasts and some clinicians use it to explain 'cold but normal thyroid labs'.

Typical reference interval: Normal reverse T3 is commonly around 90–350 pg/dL (assay-dependent). Interpretation always involves the rT3-to-free-T3 ratio and the overall thyroid picture — a standalone high rT3 value means little without the rest.. Laboratories differ — the interval printed on YOUR report always governs.

If your Reverse T3 (rT3) is high

Common reasons Reverse T3 (rT3) runs above the reference interval:

• Non-thyroidal illness or stress — the most common cause — serious illness, surgery, trauma, or major stress shift conversion toward rT3
• Starvation or very low-calorie dieting — energy deficit conservatively shifts thyroid hormone handling toward rT3
• Underactive thyroid — low conversion overall can leave inactive forms relatively higher
• Certain medications — some drugs (including certain steroids and beta-blockers) raise rT3
• Overtraining or chronic fatigue states — sustained physical stress can tilt the balance
• Laboratory variation — rT3 assays vary widely between labs — compare like with like

If your Reverse T3 (rT3) is low

Common reasons Reverse T3 (rT3) runs below the reference interval:

• Normal thyroid economy — low rT3 with normal free T3 is the healthy pattern — active hormone in use, little inactive mirror
• High-dose thyroid medication — suppression of conversion can lower rT3

Symptoms worth knowing

Reverse T3 causes no symptoms directly. The pattern it flags — high rT3 with low free T3 — may accompany persistent fatigue, cold intolerance, or sluggishness even when TSH looks unremarkable. In practice, that 'feels fine on paper but tired anyway' puzzle is the main reason people test it.

What usually happens next

1. Interpret rT3 ONLY with free T3, TSH, and free T4 — the ratio (rT3/free T3) is what matters, not the raw number
2. A high rT3 from illness or a diet phase is usually self-resolving — recovery and refeeding shift the balance back
3. For persistent fatigue with a high rT3:low free T3 pattern, a clinician (ideally thyroid-literate) reviews the full picture before any medication change

Frequently asked questions about Reverse T3 (rT3) results