Arrhythmia (a problem with the rate or rhythm of your heartbeat).Sweating or being very sensitive to heat.Weight loss, even though your appetite may increase.Symptoms of hyperthyroidism (too much thyroid hormone) may include: Heavy or irregular menstrual periods or fertility problems.Being very sensitive to cold temperatures.Symptoms of hypothyroidism (too little thyroid hormone), may include:

You may also need a T4 test if you have symptoms of having too much or too little thyroid hormone. Thyroid problems tend to run in families, so your provider may order a T4 test if a member of your family has had thyroid disease. If your TSH results aren't normal, a T4 test may be done to learn more. You may need a T4 test if your provider thinks you may have a thyroid problem. Other names: free thyroxine, free T4, total T4 concentration, T4 index, thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis

This is a less accurate way to check how well your thyroid is working. A calculation may be done to figure out how much T4 is free.

A total T4 test measures free and bound T4 together.

Medical experts believe this test provides more accurate information than a total T4 test, so it's used more often.

A free T4 test directly measures the amount of free T4 in your blood.

T4 levels can be measured with either a free T4 test or a total T4 test:

Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your tissues.

Free T4 is the active form of thyroxine hormone that enters your tissues where it's needed.

Too much or too little T4 can be a sign of thyroid disease. A T4 test measures the level of T4 in your blood. Thyroxine, also known as T4, is a type of thyroid hormone. In children, thyroid hormones affect growth, too. These hormones affect your weight, heart, body temperature, muscle strength, and even your mood. Your thyroid makes hormones that control the way your body uses energy. The thyroid is a small, butterfly-shaped gland at the base of your throat. A thyroxine test is a blood test that helps diagnose thyroid conditions.

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