Diiodothyronine

HormoneThyroid hormoneBest in the morning

What is it

Diiodothyronine (T2; specifically 3,5-diiodo-L-thyronine and 3,3'-diiodo-L-thyronine) is a thyroid hormone metabolite formed by deiodination of T3 or T4. It is sometimes marketed as a 'fat burner' or thyroid-supporting supplement.

Evidence for 1 use

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Metabolic rate / fat loss

Limited Evidence

Animal and small pilot human studies suggest increased metabolic rate and modest fat loss. Long-term safety and TSH suppression remain concerns.

How it works

Unlike T3 and T4, T2 does not bind significantly to nuclear thyroid hormone receptors. It appears to act on mitochondria, increasing oxygen consumption and uncoupling oxidative phosphorylation, which raises resting metabolic rate. Studies in animals consistently show increased thermogenesis and fat loss; small human pilot studies have suggested similar effects, though with significant suppression of TSH (a marker of thyroid axis disruption).

Dosage

Commercial supplements typically provide 100 to 300 micrograms of T2 per day. There is no medically approved dose; T2 is not a regulated drug.

When and how to take it

If used (not recommended without medical guidance), typically taken in the morning to mimic natural diurnal thyroid pattern.

2 commercial forms

Compare the main delivery options and what they’re best suited for.

3,5-Diiodo-L-thyronine (T2)

Most studied isomer; used in fat-loss supplements.

Orally bioavailable.

3,3'-Diiodo-L-thyronine

Lower activity isomer.

Less metabolically active.

Safety

T2 supplementation suppresses TSH, indicating that it is acting on the thyroid axis like a thyroid hormone. Long-term safety is unknown. Potential for arrhythmias, bone loss, anxiety, and other thyroid-excess-type symptoms, especially at higher doses or with prolonged use. Some sports supplement products containing T2 have caused thyroid laboratory disturbances in users.

Who should be cautious

Avoid in pregnancy, breastfeeding, thyroid disease, heart conditions, anxiety disorders, and osteoporosis. Not recommended for routine supplementation. People taking thyroid medication should not add T2 without medical advice.

Interactions

Likely interacts with thyroid medications (additive thyroid effect), and with drugs that affect heart rate, anxiety, and bone metabolism. Should not be combined with prescription thyroid hormones without medical supervision.

Frequently asked questions

Is T2 the same as T3 or T4?⌄

No. T2 has different receptor binding and effects. It still affects the thyroid axis but is not a standard thyroid hormone.

Is it safe to take?⌄

Long-term safety is not established. It suppresses TSH, indicating systemic thyroid effects. Use without medical supervision is risky.

References

Diiodothyronine on Wikidata — Wikidata link

Diiodothyronine on NIH DSLD (US supplement label database) — NIH Dietary Supplement Label Database link

Research on Diiodothyronine (PubMed search) — PubMed link

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Evidence-based·How we grade evidence

Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you’re pregnant, breastfeeding, on medications, or managing a chronic condition.