Optimal Ranges
Clinical (NHS) Range
FT4: 12–22 pmol/L · FT3: 3.1–6.8 pmol/L
pmol/L
Performance-Optimised Range
FT4: 15–20 pmol/L · FT3: 4.5–6.0 pmol/L
pmol/L
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.
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
Signs your levels may be off
Low / Deficiency
- Low FT3/FT4 (hypothyroid pattern):
- Fatigue despite adequate sleep
- Unexplained weight gain or inability to lose fat
- Brain fog and poor concentration
- Cold hands, feet, and intolerance to cold
- Dry skin, brittle nails, and hair thinning
- Low mood and depression
- Constipation
- Slow heart rate (bradycardia)
- Muscle weakness and joint stiffness
High / Excess
- High FT3/FT4 (hyperthyroid pattern):
- Unexplained weight loss
- Rapid or irregular heartbeat (palpitations)
- Anxiety, irritability, and tremors
- Heat intolerance and excessive sweating
- Insomnia and restlessness
- Frequent bowel movements
- Menstrual irregularity (women)
Dietary Sources
Foods that support FT3 & FT4 levels
Supplementation
Evidence-based supplementation
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.
Research
Key study
Selenium Supplementation in Patients with Autoimmune Thyroiditis Decreases Thyroid Peroxidase Antibodies Concentrations
Toulis KA, Anastasilakis AD, Tzellos TG, et al.
Journal of Clinical Endocrinology & Metabolism (2010)
DOI: 10.1089/thy.2009.0351Related Biomarkers
Related Guides
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Order Your TestThis content is for educational purposes only and does not constitute medical advice. Your data suggests areas for optimisation, but any concerns should be discussed with a qualified healthcare professional. If your results flag values outside safe ranges, we recommend consulting your GP.