What is TSH?

The pituitary tracks how much thyroid hormone is in the blood and adjusts its instruction to keep that amount steady, calling for more when the thyroid produces less. That makes TSH the first test done when thyroid disease is suspected.

FT4 is the hormone the thyroid releases once the instruction arrives. A change in TSH can begin on either side of that loop, in the gland answering or in the pituitary asking, so TSH alone does not identify where it started. FT4 measured at the same time shows what the gland did with the instruction.

The commonest reason for the thyroid's output to shift is autoimmune thyroid disease, where the immune system attacks the gland and its hormone production falls. Starting or adjusting thyroid hormone replacement adds hormone the thyroid has not made, which the pituitary reads as thyroid output and answers by calling for less. Disease in the pituitary itself weakens the instruction rather than the thyroid's ability to follow it, so TSH falls while the gland is intact.

TSH does not settle as soon as thyroid hormone output changes. Reaching its new level takes around three months, and up to six if the change in output has been running untreated for a long time. A yearly measurement is spaced beyond that settling period, so it reads a level the pituitary has reached rather than one it is still moving through. This is Do Health's monitoring schedule, not a general recommendation for everyone.