Thyroid hormoneT3 · T4 · Thyroxine · Triiodothyronine

The thyroid gland produces mostly T4 (thyroxine), which is converted in tissues to the more active T3. Together they set the pace of metabolism in essentially every cell, influencing resting energy expenditure, heart rate, body temperature, gut motility and cognition.

It matters because both directions cause real problems. An underactive thyroid brings fatigue, cold intolerance, weight gain and low mood; an overactive one brings weight loss, palpitations, anxiety and heat intolerance. Hypothyroidism is common, particularly in women, and easily treated once identified.

In practice T3 falls during sustained energy restriction — a normal adaptive response that contributes to the reduced expenditure of a long diet, and not in itself a thyroid disorder. Persistent symptoms deserve a blood test rather than a supplement, since TSH is the first-line screen.