Hormones

Free T3 and Free T4

The active and storage forms of thyroid hormone your labs should show

Plain English

Free T3 (triiodothyronine) and Free T4 (thyroxine) are the two active thyroid hormones circulating in the blood, unbound and available to enter cells. T4 is the storage form, produced in large quantities by the thyroid gland. T3 is the active form, converted from T4 in peripheral tissues, and is roughly four times more potent. TSH alone does not tell you whether the conversion step is working.

The Mechanism

The thyroid gland produces about 80% T4 and 20% T3 in response to thyroid-stimulating hormone (TSH) from the pituitary. T4 circulates bound to carrier proteins, with a small fraction (roughly 0.03%) remaining free and bioavailable. This free fraction is what Free T4 measures. T4 is converted to the more potent T3 in the liver, kidneys, and other peripheral tissues by enzymes called deiodinases. Free T3 reflects the output of this conversion step.

T3 regulates metabolism at the cellular level by entering the nucleus and directing gene expression for energy production, protein synthesis, heart rate, body temperature, and gut motility. When T3 is low, every metabolic process slows: basal metabolic rate drops, muscle protein synthesis decreases, cardiac output falls, and cognition clouds. This is why people with normal TSH but poor T4-to-T3 conversion can experience every symptom of hypothyroidism while a standard thyroid panel looks unremarkable.

Conversion efficiency is affected by several factors: chronic stress and elevated cortisol, selenium and zinc deficiency (both required by the deiodinase enzymes), severe calorie restriction, and systemic inflammation. This is the practical reason that aggressive dieting sometimes produces thyroid-like symptoms: T4 is available, but conversion to T3 slows as the body conserves energy.

Why It Matters

TSH tells you the pituitary is trying. Free T3 tells you whether the thyroid is actually delivering.

Thyroid hormones set the speed of almost every metabolic process in the body. Low Free T3 explains persistent fatigue, cold intolerance, slow recovery, and difficulty losing body fat even at appropriate calorie deficits. Because TSH normalizes before Free T3 does after treatment or lifestyle correction, Free T3 is the marker that most accurately reflects whether thyroid function has actually recovered. If you have thyroid symptoms with a normal TSH, request Free T3 and Free T4 separately.

Common Misconception

Most clinicians order only TSH for thyroid assessment, and a normal result is often interpreted as ruling out thyroid dysfunction. TSH reflects pituitary signaling, not thyroid output or T4-to-T3 conversion. A person with normal TSH, normal T4, and poor peripheral conversion can have functionally low T3 and every symptom of hypothyroidism with a clean standard panel.

Signs It Is Disrupted

Persistent fatigue and low energy that does not respond to adequate sleep
Cold hands, feet, or consistently lower basal body temperature
Hair thinning or hair loss, particularly at the outer third of the eyebrows
Slow recovery between training sessions, more than expected for the load
Difficulty losing weight despite appropriate calorie deficit and adequate protein intake
Constipation, slow digestion, or persistently sluggish gut motility
Brain fog, low mood, or cognitive dulling without obvious cause

How to Improve It

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Adequate selenium. Selenium is required by the deiodinase enzymes that convert T4 to T3; two Brazil nuts per day provides roughly 55-70 mcg, or targeted supplementation at 100-200 mcg covers the requirement for most people.

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Adequate zinc. Zinc deficiency impairs both thyroid hormone synthesis and T4-to-T3 conversion; oysters, red meat, and pumpkin seeds are dense dietary sources, with 15-30mg supplemental zinc as an alternative.

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Avoid severe calorie restriction. Aggressive calorie deficits trigger a protective reduction in T3 output and peripheral conversion; moderate deficits of 400-500 calories below maintenance preserve thyroid function far better than large ones.

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Manage cortisol load. Chronic HPA axis activation suppresses T4-to-T3 conversion directly; Zone 2 training, adequate sleep, and stress reduction all support conversion efficiency.

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Test comprehensively. A complete thyroid panel includes TSH, Free T4, Free T3, and thyroid antibodies (TPO, TgAb) to distinguish conversion problems from autoimmune thyroiditis such as Hashimoto's disease.

3 Things to Remember

Free T4 is the thyroid storage form; Free T3 is the active form that drives metabolism. A normal TSH does not confirm that conversion between the two is working.

Low Free T3 produces fatigue, cold intolerance, slow recovery, and impaired fat loss even when TSH and T4 appear normal on a standard panel.

Selenium, zinc, moderate calorie intake, and cortisol management are the four primary lifestyle inputs that support healthy T4-to-T3 conversion.

Appears In

→What Your Thyroid Numbers Mean for Energy, Recovery, and Body Composition →The Lab Work & Biomarkers Protocol →The Stress & Cortisol Protocol

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