Free T3: what your result means.

The most active thyroid hormone; converts from T4 in tissues. Free T3 is measured by blood test as part of the Hormonal panel in the HodieLabs Health Blueprint.

Why Free T3 matters

FT3 is 3–4× more metabolically active than FT4 at the cellular level. Low T3 syndrome (sick euthyroid) occurs in severe illness, caloric restriction, and high cortisol states.

Associated risks when out of range: Thyroid dysfunction. Metabolic symptoms (contextual).

Evidence basis

The HodieLabs Evidence-Based Clinical Library classifies Free T3 as an strong evidence-tier biomarker. Its reference ranges come from a published authority standard rather than from observational optimisation targets. The standard used here is American Thyroid Association (ATA) 2017; free T3 3.5–6.5 pmol/L reference. Where relevant clinical outcomes are noted on this page, they reflect the guideline literature referenced below.

Evidence-based reference ranges

The HodieLabs Evidence-Based Clinical Library interprets Free T3 against published clinical authority standards, never arbitrary "optimal" ranges. Measured in pmol/L:

Clinical interpretation

• FT3 is 3–4× more metabolically active than FT4 at the cellular level.
• Low T3 syndrome (sick euthyroid) occurs in severe illness, caloric restriction, and high cortisol states.
• Selenium and zinc are required cofactors for T4-to-T3 conversion (deiodinase enzymes).
• Biotin supplements can interfere with thyroid immunoassays (ATA/ADLM). Washout 8–72 hours depending on dose and assay platform.

What improves it: the evidence-based levers

• FT3 is the most metabolically active thyroid hormone. Reflects actual tissue thyroid activity
• Low FT3 with normal FT4 may indicate poor T4-to-T3 conversion (selenium, zinc, or stress-related)

Recommended testing frequency: Annual, as part of a comprehensive panel. Single markers are always interpreted in context.

How Free T3 testing works at HodieLabs

Free T3 is measured in every Health Blueprint at our Melbourne Preventative Health & Longevity Clinic, interpreted by a doctor against the guideline-cited ranges above and tracked over time through your HodieLabs membership.

References

1. American Thyroid Association (ATA) 2017; free T3 3.5–6.5 pmol/L reference.

This page is general information from the HodieLabs Evidence-Based Clinical Library, not medical advice. Speak with your GP about what testing and care is appropriate for you.