Free T3 & Free T4 (Thyroid Hormones)

Optimal ranges

What is the optimal range for FT3 & FT4?

The clinical range defines what is considered medically “normal” — broad enough to cover 95% of the population. The performance range reflects where research and clinical experience suggest most people feel and function at their best. A result in either range suggests typical status and is not a diagnosis; any individual reading should be interpreted by a qualified clinician.

Why it matters

Why FT3 & FT4 matters for performance

TSH alone is a screening test, not a diagnosis. It tells you the pituitary is shouting at the thyroid, but not what the thyroid is actually producing. You can have a normal TSH with low Free T3 — a pattern called 'low T3 syndrome' or 'euthyroid sick syndrome' — which produces classic hypothyroid symptoms (fatigue, weight gain, brain fog, cold hands) that a GP may dismiss because TSH looks fine. Free T4 can also be normal while Free T3 is low, indicating poor T4-to-T3 conversion — a common issue in chronic stress, caloric restriction, selenium deficiency, and gut inflammation. Testing both Free T3 and Free T4 alongside TSH gives a complete picture of thyroid output and conversion efficiency. For active people, even mildly low Free T3 impairs exercise recovery, fat metabolism, and mitochondrial energy production.

Symptoms

What are the symptoms of low or high FT3 & FT4?

Dietary sources

Which foods support FT3 & FT4 levels?

Supplementation

How do you improve FT3 & FT4 levels?

Selenium (200 mcg/day as selenomethionine) is the most evidence-based nutrient for thyroid hormone conversion — it is a cofactor for the deiodinase enzymes that convert T4 to T3. Studies show selenium supplementation improves FT3:FT4 ratios and reduces thyroid antibodies in Hashimoto's thyroiditis. Iodine (150–300 mcg/day) only if deficiency is confirmed via urinary iodine — excess iodine can paradoxically suppress thyroid function (Wolff-Chaikoff effect). Zinc (15–30 mg/day) supports TSH receptor sensitivity and T3 binding to nuclear receptors. Iron should be optimised (ferritin >50 µg/L) as iron deficiency impairs thyroid peroxidase, the enzyme that synthesises T3 and T4. Ashwagandha (600 mg KSM-66 daily) has shown modest improvements in TSH and T4 levels in subclinical hypothyroidism. Avoid excessive raw cruciferous vegetables (goitrogens) and soy isoflavones if thyroid function is compromised — cooking neutralises most goitrogenic compounds. If FT4 is low or TSH is persistently above 10 mIU/L, GP referral is essential. Levothyroxine (synthetic T4) is the standard treatment for hypothyroidism; liothyronine (synthetic T3) is occasionally added when conversion is impaired, though UK NHS prescribing of T3 is limited. Retest thyroid panel at 6–8 weeks after any intervention.

Testing

How is FT3 & FT4 tested in the UK?

FT3 & FT4 is measured from a blood sample. With Helvy, that means a finger-prick kit taken at home and posted to a UKAS-accredited UK laboratory, with results in around 5 days, reviewed by a qualified clinician. Your result is reported against both the clinical range (FT4: 12–22 pmol/L · FT3: 3.1–6.8 pmol/L) and the performance-optimal range (FT4: 15–20 pmol/L · FT3: 4.5–6.0 pmol/L), so you can see not just whether you are “normal” but whether you are optimal. If you make a change, retest after 8-12 weeks to confirm it worked.

Research

Key study