Free T4 (Thyroxine)

Your thyroid is a small, butterfly-shaped gland in the front of your neck. It makes hormones that control the way your body uses energy.

T4 is the main hormone that your thyroid makes. There are two forms of it in your blood: Free T4 is the active form of thyroxine hormone that enters your body tissues where you need it. Bound T4 is thyroxine that attaches or binds to certain proteins which prevent it from entering your body tissues.

In general, T4 results that are lower than normal may be a sign of hypothyroidism, Hashimoto's disease, certain stages of thyroiditis, a pituitary problem (uncommon), getting too much or too little iodine in your diet, or congenital hypothyroidism.

In general, T4 results that are higher than normal may be a sign of hyperthyroidism, Graves' disease, certain stages of thyroiditis, toxic goiter, toxic thyroid nodule, too much iodine, a noncancerous (not cancer) tumor in your pituitary gland, taking too much thyroid hormone medicine to treat hypothyroidism, or a pituitary problem (uncommon).