Setting your metabolic pace

Clinically: Thyroid hormone regulation

What normal looks like

A working thyroid keeps hormone levels remarkably steady day to day. The pituitary constantly adjusts its signal to hold the level where it should be, which is why that signal (TSH) changes before the hormones themselves do.

unverified These explanations are written to be easy to read and have not yet been checked against a source. The ranges and measurement notes on this page show theirs.

function unverified

Your thyroid sets how fast your body runs. A control centre in your brain watches the level and sends a signal to speed it up or slow it down. That signal shifts before the hormone itself does, which is why it is tested first.

TSH from the anterior pituitary drives thyroid hormone output, and circulating free T4 suppresses TSH in turn. The loop's sensitivity means TSH moves earlier and more than free T4, making it the preferred first-line test.

In clinical terms

Hypothalamic-pituitary-thyroid feedback loop setting circulating thyroxine and triiodothyronine, and through them basal metabolic rate.