The big picture: Free T4 (thyroxine) is a crucial hormone produced by the thyroid gland that plays a key role in metabolism, energy regulation, and overall thyroid function. While T4 is inactive on its own, it is converted into T3 (triiodothyronine), the active hormone responsible for regulating many bodily processes. Free T4 measures the unbound form of T4 in the bloodstream, which is available for conversion into T3. This test is essential for assessing thyroid function and diagnosing thyroid-related disorders.

Role in thyroid function: T4 serves as a precursor to T3, the more active thyroid hormone. While the thyroid produces both T3 and T4, about 90% of the hormone secreted is T4, which must be converted into T3 in tissues like the liver and kidneys. Free T4 provides insight into how well the thyroid is producing hormones, while free T3 shows how much active hormone is available to regulate metabolism.

Low free T4 (hypothyroidism): Low levels of free T4 indicate an underactive thyroid, leading to symptoms such as fatigue, weight gain, cold intolerance, constipation, dry skin, and depression. This is commonly seen in conditions like Hashimoto’s thyroiditis. Treatment usually involves thyroid hormone replacement therapy, such as levothyroxine, to restore normal levels.

High free T4 (hyperthyroidism): Elevated free T4 levels suggest an overactive thyroid, often seen in conditions like Graves’ disease. Symptoms include weight loss, anxiety, tremors, sweating, rapid heart rate, and heat intolerance. Treatment may include medications to reduce thyroid activity, radioactive iodine, or surgery in severe cases.

Free T4 vs Free T3: Free T4 measures the precursor hormone that must be converted into T3 for the body to use. Free T3 is the more biologically active hormone that directly regulates metabolism. While free T4 reflects hormone production, free T3 provides a clearer picture of how well the body is using thyroid hormones.