Thyroid function tests are a series of blood tests used to measure health of thyroid gland. The thyroid produces two major hormones: triiodothyronine (T3) and thyroxine (T4). If your thyroid gland doesn’t produce enough of these hormones, you may experience symptoms such as weight gain, lack of energy, and depression. This condition is called hypothyroidism.If your thyroid gland produces too many hormones, you may experience weight loss, high levels of anxiety, tremors, and a sense of being on a high. This is called hyperthyroidism.