Active thyroid hormone (nicknamed T3, short for triiodothyronine) sets the body’s metabolic rate, sort of like a volume dial. One might say T3 determines how hard or how fast each cell works to do its job. Every cell of the body is affected by T3.

The thyroid glands (there’s one gland on each side of the windpipe) do not produce T3. Instead, they produce an inactive form called T4. Tissues of the body absorb T4 and convert it to T3.

(original graphic by marvistavet.com)