The butterfly-shaped organ at the base of your neck plays a pivotal role in your body's metabolism. It regulates the release of hormone production and metabolism of your body. The active form of thyroid hormone is T3, not TSH or T4. T3 is converted from T4 hormones in the gut and lever.
Then what is TSH for? It is a hormone produced by the pituitary gland. It controls the function of the thyroid gland. The levels of TSH get affected earliest when there is any thyroid disease. This is why some doctors advise getting only TSH levels done.