A thyroxine test helps diagnose disorders of the thyroid. The thyroid is a small, butterfly-shaped gland located near the throat. Your thyroid makes hormones that regulate the way your body uses energy. It also plays an important role in regulating your weight, body temperature, muscle strength, and even your mood. Thyroxine, also known as T4, is a type of thyroid hormone. This test measures the level of T4 in your blood. Too much or too little T4 can indicate thyroid disease.

The T4 hormone comes in two forms:

• Free T4, which enters the body tissues where it's needed
• Bound T4, which attaches to proteins, preventing it from entering body tissues

A test that measures both free and bound T4 is called a total T4 test. Other tests measure just free T4. A free T4 test is considered more accurate than a total T4 test for checking thyroid function.

Other names: free thyroxine, free T4, total T4 concentration, thyroxine screen, free T4 concentration.

Thyroid disease is much more common in women and most often occurs under the age of 40. It also tend to run in families. You may need a thyroxine test if a family member has ever had thyroid disease or if you have symptoms of having too much thyroid hormone in your blood, a condition called hyperthyroidism, or symptoms of having too little thyroid hormone, a condition called hypothyroidism.

Symptoms of hyperthyroidism, also known as overactive thyroid, include:

• Anxiety
• Weight loss
• Tremors in the hands
• Increased heart rate
• Puffiness
• Bulging of the eyes
• Trouble sleeping

Symptoms of hypothyroidism, also known as underactive thyroid, include:

• Weight gain
• Fatigue
• Hair loss
• Low tolerance for cold temperatures
• Irregular menstrual periods
• Constipation

Your results may come in the form of total T4, free T4, or a free T4 index.

• The free T4 index includes a formula that compares free and bound T4.
• High levels of any of these tests (total T4, free T4, or free T4 index) may indicate an overactive thyroid, also known as hyperthyroidism.
• Low levels of any of these tests (total T4, free T4, or free T4 index) may indicate an underactive thyroid, also known as hypothyroidism.

If your T4 test results are not normal, your health care provider will likely order more thyroid tests to help make a diagnosis. These may include:

• T3 thyroid hormone tests. T3 is another hormone made by the thyroid.
• A TSH (thyroid stimulating hormone) test. TSH is a hormone made by the pituitary gland. It stimulates the thyroid to produce T4 and T3 hormones.
• Tests to diagnose Graves' disease, an autoimmune disease that causes hyperthyroidism
• Tests to diagnose Hashimoto's thyroiditis, an autoimmune disease that causes hypothyroidism

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