FT4 or free T4 test iso help diagnose and monitor treatment of thyroid disorders. Thyroid stimulating hormone is the preferred initial test in the assessment of thyroid function. Free thyroxine (FT4) may be measured in response to an abnormal TSH test result.TSH stands for thyroid stimulating hormone. A TSH test is a blood test that measures this hormone. TSH levels that are too high or too low may be a sign of a thyroid problem. The thyroid is a small, butterfly-shaped gland in the front of your neck. Your thyroid makes hormones that control how your body uses energy. Thyroid hormones affect nearly every organ in your body, including your heart. They help control your weight, body temperature, muscle strength, and even your mood. If you don't have enough thyroid hormones in your blood, many of your body functions slow down. If you have too much, many body functions speed up.