The test is also known as T4 or free thyroxine. Thyroxine-Free and a combination of triiodothyronine (T3) together produce thyroid gland. It is through T4 that the body control rate is measured. It is all regulated in a feedback system. The doctor may order the T4 test if the doctor sees the abnormal result on a TSH test or suspects of hyperthyroidism or hypothyroidism. If the patient shows increased high rates, anxiety, sudden weight loss or being the insomnia, tremors in the hands or have visual disturbances, he can be a suspect for hyperthyroidism, and the T4 test can be called in for. Whereas weight gains, dry skin, cold intolerance, fatigue, hair loss, irregularity in menstruation cycle of women, constipation can be symptoms of hypothyroidism. 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.