We’ve mentioned reverse T3 a few times across this site, and it deserves its own explanation — because for some people, it’s the missing piece that finally explains “normal labs, still unwell.”
What reverse T3 is
Your thyroid makes T4 (the storage form). Your body converts T4 into T3 (the active form that your cells use). But there’s a third possibility: your body can convert T4 into reverse T3 (rT3) — a mirror-image molecule that fits into the T3 receptor like the wrong key in a lock. It occupies the space without turning anything on.
When does your body make more rT3?
Reverse T3 production increases in response to stress, illness, inflammation, and caloric restriction — all situations where your body slows down non-essential processes to conserve energy. For people with Hashimoto’s, who often carry an ongoing inflammatory load, this pathway can be chronically upregulated.
The mechanism: the same deiodinase enzymes that convert T4 into active T3 can instead shunt T4 toward rT3. Selenium is required for those enzymes to function — another reason selenium deficiency matters for thyroid conversion (Larsen & Zavacki, 2013).
Is reverse T3 testing routine?
No — rT3 testing is not part of standard care and its interpretation is debated among endocrinologists. Some practitioners use it as part of a broader picture; others don’t find it clinically actionable. We include it here because understanding why your cells might not be getting the active hormone matters — even if the test itself isn’t the focus.
What is actionable: supporting the deiodinase enzymes (via adequate selenium), reducing inflammatory load, and addressing the antibody attack — all of which aim at the same root.
Frequently asked questions
What is reverse T3? Reverse T3 (rT3) is an inactive, mirror-image form of thyroid hormone your body can make from T4 instead of active T3. It occupies space without driving metabolism (Bianco).
What causes high reverse T3? Illness, inflammation, chronic stress (high cortisol), aggressive caloric restriction, and deficiencies like low iron can all push conversion toward reverse T3 (PMC).
Can reverse T3 explain symptoms with normal labs? It can be part of it — a pattern of high rT3 with low free T3 but normal TSH may mean too much of your hormone is inactive. But rT3 interpretation is debated, so treat it as one lens, not the whole answer.
How do you lower reverse T3? By reducing what drives it up — chronic stress and crash dieting — and supporting healthy conversion (correcting deficiencies, adequate selenium), rather than targeting the number directly.
