The thyroid produces T4 (thyroxine) and a smaller amount of T3 (triiodothyronine). T3 is the active form — it enters cells and directly controls metabolic rate by influencing gene expression. Every cell in your body has thyroid receptors: heart, brain, gut, muscles, bones, and skin. When T3 is optimal, you have energy, mental clarity, healthy weight, and warmth.
The system is regulated by a feedback loop: the hypothalamus releases TRH, the pituitary releases TSH (thyroid-stimulating hormone), and the thyroid responds by producing T4. T4 is converted to T3 primarily in the liver and gut. Disruption at any point — thyroid damage, poor conversion, nutrient deficiency, or autoimmune attack — can impair the entire system.
Standard thyroid testing often measures only TSH — and uses outdated reference ranges. A TSH of 4.0 mIU/L is considered 'normal' by many labs, but functional medicine practitioners argue that optimal is 1.0-2.5. Many people with TSH between 2.5 and 4.5 have symptoms of hypothyroidism that are dismissed because their numbers are 'within range.'
More importantly, TSH alone doesn't tell you about T3 levels — the hormone that actually acts on cells. You can have normal TSH with poor T4-to-T3 conversion, leaving your cells hypothyroid despite normal-looking blood work. A complete thyroid panel should include TSH, Free T4, Free T3, thyroid antibodies (TPO and TG), and reverse T3. Hashimoto's thyroiditis (autoimmune thyroid disease) is the most common cause of hypothyroidism and requires antibody testing to diagnose.
Four nutrients are critical for thyroid function: Selenium (needed for T4-to-T3 conversion; 200mcg daily or 2-3 Brazil nuts), iodine (the raw material for thyroid hormones; 150-300mcg daily from sea vegetables, fish, or iodised salt), zinc (required for TSH receptor function), and iron (needed for thyroid peroxidase enzyme activity).
Avoid goitrogens (compounds that interfere with thyroid function) in excess: raw cruciferous vegetables, soy, and millet can inhibit iodine uptake when consumed in large amounts — cooking largely deactivates these compounds. Gluten may exacerbate Hashimoto's through molecular mimicry. Manage stress (cortisol impairs T4-to-T3 conversion) and support gut health (20% of T4-to-T3 conversion happens in the gut). If you suspect thyroid issues, insist on a full panel — TSH alone is not enough.
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