Thyroid Stimulating Hormone (TSH)
The anterior pituitary gland secretes TSH in response to the amount of T4 & T3 hormones in the blood. Just as the name implies, TSH stimulates the thyroid gland to secrete thyroid hormones. The higher the levels of T3 & T4 the less the body needs so TSH levels decline. When T3 & T4 levels are low TSH levels increase trying to stimulate more hormone production from the thyroid gland. This is called a negative feedback system.
I remember being taught in medical school that TSH is all you need in order to evaluate thyroid function. However, nothing could be further from the truth. The TSH gives us an idea of your thyroid function but it is far too simplistic to say that it is a thorough evaluation.