Deiodinases: How Your Body Turns Levothyroxine Into Active T3

Levothyroxine (T4) is a prohormone — your body activates it by stripping off iodine atoms with three selenium-containing enzymes called deiodinases. Most people convert just fine on levothyroxine alone, but a subgroup with persistent symptoms has spurred ongoing research into DIO2 polymorphisms and T4/T3 combination therapy. Evidence is mixed; current guidelines still favor levothyroxine monotherapy first.

T4 is a prohormone

The thyroid gland mostly secretes thyroxine (T4), a hormone with four iodine atoms. T4 is largely inactive on its own. The hormone that actually binds to receptors in your cells and turns on metabolism is triiodothyronine (T3) — T4 with one iodine atom removed [C2].

That removal is done by enzymes called deiodinases, and almost all of the T3 circulating in your body — and most of the T3 inside individual tissues — comes from this conversion, not directly from the thyroid [C2]. Levothyroxine works because once you swallow the tablet, your own deiodinases pick up where the thyroid left off and produce the T3 your cells need.

The three deiodinases: D1, D2, D3

There are three deiodinase enzymes, each with a different job and a different tissue distribution [C2][C7]:

• D1 (DIO1) sits mainly in the liver, kidneys, and thyroid. It contributes to the T3 you see in a blood test. It also helps clear reverse T3 and inactive metabolites.
• D2 (DIO2) is the local converter. It lives inside the brain, pituitary, brown fat, muscle, and several other tissues. D2 is what allows a brain cell or a pituitary cell to make its own T3 from circulating T4 — independent of what the liver is doing.
• D3 (DIO3) is the off switch. It inactivates T4 and T3 (converting them to reverse T3 and T2) so tissues can ramp hormone signaling down when they need to — for example, during illness, fasting, or fetal development [C2][C6].

All three are selenoproteins — they require a selenium-containing amino acid (selenocysteine) at their active site to work. This is part of why selenium and thyroid biology are linked, although that does not mean supplementing selenium beyond adequate intake fixes conversion problems [C7]. See our selenium-hashimotos article.

Why we measure TSH, not T3

A common patient question: "If T3 is the active hormone, why doesn't my doctor measure it?"

The answer is in the pituitary. The pituitary gland — which produces TSH — has lots of D2. It reads local T3, generated on the spot from circulating T4, and adjusts TSH up or down accordingly [C1][C2]. TSH is therefore an integrated signal of how well thyroid hormone is reaching that tissue. A normal TSH on levothyroxine generally means the pituitary is satisfied that enough T4 is around to keep its own T3 supply normal.

Total or free T3 in the blood is a weaker signal because most tissues make their own T3 locally; the circulating number doesn't necessarily reflect what's happening inside the brain or muscle [C2]. This is why the American Thyroid Association and other major societies recommend TSH (with free T4 if needed) as the primary monitoring lab [C1].

"I feel terrible at a normal TSH" — what we know

Most patients on levothyroxine feel well. But surveys consistently find a subgroup — roughly 10–15% — who report persistent fatigue, brain fog, or low mood despite a normal TSH [C1][C4]. This has driven decades of research into whether some people convert T4 to T3 less efficiently, and whether they would do better with a little T3 added to their regimen.

The most studied candidate is the DIO2 Thr92Ala polymorphism, a common variation in the gene that codes for D2 [C3][C4]. Carriers may have slightly altered D2 enzyme activity. Some studies have linked it to subjective well-being on levothyroxine, possible response to combination therapy, and associations with body weight, glucose, and cognitive aging [C3][C4]. But the literature is genuinely mixed: many studies find no clear clinical effect, and large randomized trial meta-analyses of T4/T3 combination therapy have not shown a consistent benefit on symptoms or quality of life over levothyroxine alone [C1][C4][C5].

The current picture: the polymorphism may matter for a subset of people, but we can't yet reliably predict who, and routine DIO2 testing is not recommended [C1][C4][C5].

Reverse T3 (rT3): why most endocrinologists don't routinely test it

When D3 cuts an iodine off T4 in a different position than D2 does, you get reverse T3 (rT3) — a metabolically inactive isomer. Reverse T3 rises during illness, starvation, severe stress, and in critically ill patients ("non-thyroidal illness syndrome") [C6].

This has fueled an online narrative that "high rT3" or a "low T3/rT3 ratio" explains persistent symptoms. The reality is more boring [C1][C6]:

• rT3 is a marker of illness and acute stress, not a treatable condition on its own. Treating the underlying illness — not the rT3 number — is what brings it down [C6].
• The T3/rT3 ratio is not clinically actionable by current evidence; the ATA does not recommend routine rT3 testing or T3 supplementation based on rT3 levels [C1].
• A 2026 narrative review on rT3 in neuropsychiatric disorders found associations in specific conditions but did not establish that lowering rT3 changes outcomes [C6].

If your endocrinologist hasn't ordered an rT3, it's not because they missed something — it's because the result wouldn't change management [C1].

T4/T3 combination therapy: state of the evidence

The 2021 American Thyroid Association consensus document spelled it out [C5]: combination therapy (levothyroxine plus liothyronine, the synthetic T3) may be considered in carefully selected patients who remain symptomatic on adequate levothyroxine after other causes have been ruled out — but it should not be first-line, the T4:T3 ratio should approximate physiologic (~13:1 to 16:1), and patients need to be monitored for over-replacement (suppressed TSH, palpitations, atrial fibrillation, bone loss) [C5][C8].

The European Thyroid Association is somewhat more open to trial combination therapy in this subgroup, but agrees that monotherapy is the default [C4][C5]. Desiccated thyroid extract (pig-derived) delivers a non-physiologic T4:T3 ratio (~4:1) and is not endorsed by the ATA [C1][C5].

What does NOT help

Several approaches popular online lack supporting evidence [C1][C5][C7]:

• Switching to desiccated thyroid without a specific indication. The T3 dose is non-physiologic and harder to titrate [C1][C5].
• T3-only therapy in standard hypothyroidism. Short half-life produces large hormone swings; not a first-line option [C5].
• Megadose selenium beyond 200 mcg/day "to boost conversion." Deiodinases are seleno-enzymes, but in selenium-replete people, more selenium doesn't increase T3 output and high intakes carry real toxicity risk [C7].
• Routine rT3 testing or "low T3 syndrome" treatment in otherwise stable patients [C1][C6].

Practical guidelines

1. TSH + free T4 first. These remain the standard for monitoring levothyroxine treatment [C1].
2. Get selenium intake to adequacy, not excess. Adequate dietary selenium supports deiodinase function; supplementation beyond ~200 mcg/day is not supported for "boosting conversion" [C7]. See selenium-hashimotos.
3. Rule out the common stuff first. Persistent symptoms at a normal TSH warrant checking iron/ferritin, vitamin D, B12, sleep apnea, depression, and reviewing the levothyroxine routine (empty stomach, timing) before invoking conversion problems [C1][C8].
4. Discuss combination therapy with your endocrinologist if symptoms persist after that workup. It's a legitimate option in carefully selected patients, not a first move [C5].
5. Don't test rT3 expecting actionable results. The number rarely changes the management plan [C1][C6].

Frequently asked questions

Does levothyroxine work if my D2 is "weak"? For most people, yes — the body has redundancy across D1, D2, and D3, and the TSH/D2 feedback loop keeps tissue T3 in range [C2]. The DIO2 Thr92Ala polymorphism has modest effects in some studies and none in others; it's not currently a routine clinical test [C3][C4].

Should I get a reverse T3 test? Generally no. The ATA does not recommend it in routine hypothyroidism management. rT3 reflects illness, stress, or fasting and doesn't change treatment [C1][C6].

Will adding T3 (liothyronine) make me feel better? For most patients, no — randomized trials and consensus reviews don't show consistent quality-of-life improvement over levothyroxine alone. But a minority of patients does report benefit, and combination therapy is a reasonable trial in carefully selected, persistently symptomatic patients with normal labs and other causes excluded [C5]. See liothyronine-t3-only.

Does selenium fix conversion problems? Adequate selenium intake supports deiodinase function. In selenium-replete people, supplementing more does not measurably improve T4-to-T3 conversion or symptoms; high doses are toxic [C7]. See selenium-hashimotos.

Is "T3 dominance" or "low T3 syndrome" something I should be treated for? Not in standard outpatient hypothyroidism. Low T3 with normal TSH usually reflects illness, calorie restriction, or recent stress and resolves with treating the underlying issue [C1][C6].

Bottom line

Levothyroxine works because your own deiodinase enzymes convert T4 to active T3 in your tissues [C2]. Most patients on levothyroxine convert well and feel normal at a normal TSH [C1]. A subgroup has persistent symptoms despite normal labs, and DIO2 polymorphism research is trying to explain why — but evidence is mixed, large meta-analyses of T4/T3 combination therapy have not consistently outperformed monotherapy, and the ATA still recommends levothyroxine first-line [C1][C3][C4][C5]. Reverse T3 testing is not routinely useful [C1][C6]. If you're symptomatic at a normal TSH, work through the common contributors first (iron, vitamin D, sleep, medication routine) and discuss combination therapy with your endocrinologist as a considered next step, not a default [C5][C8].

Sources

1. [C1] Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism. Thyroid. 2014;24(12):1670–1751. PubMed: 25266247
2. [C2] Köhrle J. Deiodinases control local cellular and systemic thyroid hormone availability. 2022. PubMed: 36206932
3. [C3] Bing Y. The type 2 deiodinase Thr92Ala polymorphism and cognitive aging: current evidence and biological context. 2026. PubMed: 42137609
4. [C4] Penna GC. Gene polymorphisms and thyroid hormone signaling: implication for the treatment of hypothyroidism. 2024. PubMed: 37740833
5. [C5] Jonklaas J et al. Evidence-Based Use of Levothyroxine/Liothyronine Combinations in Treating Hypothyroidism: A Consensus Document. 2021. PubMed: 33777817
6. [C6] Gordon ML. The Influence of Reverse Triiodothyronine on Neuropsychiatric Disorders: A Narrative Review. 2026. PubMed: 41168656
7. [C7] Lee KW. Inherited Disorders of Thyroid Hormone Metabolism Defect Caused by the Dysregulation of Selenoprotein Expression. 2021. PubMed: 35126314
8. [C8] Baskaran BS et al. Risk of cardiac, neuropsychiatric and musculoskeletal adverse events with levothyroxine: Systematic review. 2026. PubMed: 41559017

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For educational purposes only. Not medical advice. Always consult your healthcare provider.