Just What is TSH Anyway?

TSH is short for thyroid stimulating hormone. It is a hormone produced by your master gland, the pituitary. It's purpose is to stimluate the production of thyroid hormone by the thyroid. The higher it is the more stimulation it provides. It serves a negative feedback loop to the thyroid. So when TSH levels are low it means the thyroid is generally producing enough thyroid hormone. When TSH levels are high, this usually means the body requires more thyroid hormone.

What About T3 and T4?

These are actually hormones that the thyroid gland produces. It also produces T1 and T2 but these are not generally addressed. The number in the name actually refers to the number of iodine molecules present in the hormone.

T4 is what the synthetic hormones like Synthroid have. It is a pre-hormone and the body actually converts it to the active hormone T3 which performs most of the activity of the thyroid function.