The Free T4 test is used to assess thyroid function, indicate hypothyroidism or hyperthyroidism, help diagnose thyroid disease, and evaluate the effectiveness of thyroid treatment.

Thyroxine (T4) is one of the two main hormones produced by the thyroid, a butterfly-shaped gland in your throat. Along with the hormone Triiodothyronine (T3), T4 helps regulate how your body stores and uses energy. Thyroxine affects various areas of health, including metabolism, growth, digestion, heart and muscle function, brain development, and bone maintenance.

Thyroxine exists in two forms: bound and free. Bound thyroxine attaches to proteins. Free thyroxine (FT4) is the active form of thyroxine and is not bound to proteins in the blood, which allows it to travel into organs that use T4. Fluctuations in protein levels do not affect free T4, so free T4 is a more reliable measurement of T4 levels in the blood.

By seeing if you are in the normal range, the free T4 test can help detect if there is too much (hyperthyroidism) or too little thyroxine (hypothyroidism), aid in the diagnosis of pituitary disorders and female infertility, and monitor thyroid hormone treatment.  

When the thyroid, pituitary, hypothalamus are functioning normally, the body’s feedback system maintains a relatively stable and normal ratio of thyroid hormones. When the hormone balance is disrupted with an excess or deficiency of T4, the body tries to counterbalance and restore the normal ratio. When thyroid hormone levels decrease in the blood, the pituitary gland produces TSH (thyroid stimulating hormone), which bind to the TSH receptor and stimulate the thyroid to produce and release T4 and T3. Alternatively, when thyroid hormone levels increase in the blood, the pituitary gland decreases its production of TSH so the thyroid produces less T4 and T3.

Symptoms of hyperthyroidism and high T4 levels can include:

• Weight loss
• Increased heart rate
• Anxiety
• Trouble sleeping
• Tremors in the hands
• Weakness
• Diarrhea
• Light sensitivity, visual disturbances
• Puffiness around the eyes, dryness, irritation
• Bulging eyes

Symptoms of hypothyroidism and low T4 levels can include:  

• Weight Gain
• Enlarged thyroid (goiter)
• Fatigue
• Hair Loss
• Constipation
• Irregular menstrual periods in women
• Intolerance to cold
• Puffy skin

Hashimoto’s thyroiditis is the most common cause of hypothyroidism. The free T4 test, along with other thyroid tests such as  TSH and Thyroperoxidase (TPO) tests, help determine this disorder.