The free thyroxine (FT4) test measures the unbound thyroid hormone thyroxine in blood to assess thyroid gland function. It helps diagnose and monitor hypothyroidism or hyperthyroidism, guides dose adjustments for thyroid replacement or antithyroid therapy, and—together with TSH—evaluates pituitary or central thyroid disorders. It’s used when symptoms like fatigue, weight change, palpitations, or pregnancy-related concerns arise.

Low FT4 means reduced free thyroxine in the blood, indicating decreased thyroid activity. It commonly reflects primary hypothyroidism (thyroid gland failure) but can stem from pituitary or hypothalamic issues. Symptoms include fatigue, weight gain, cold intolerance, dry skin, constipation, and slow heart rate. Doctors check TSH and antibody tests to identify the cause and may treat with levothyroxine replacement.

High T4 means elevated thyroid hormone levels, usually indicating an overactive thyroid (hyperthyroidism). Common causes include Graves’ disease, toxic nodules, thyroiditis, or excess thyroid medication. Symptoms may include weight loss, rapid heartbeat, heat intolerance, anxiety, tremors, and fatigue. Typically TSH is low when T4 is high. Evaluation by a doctor is important to confirm the cause and guide treatment such as medication, radioactive iodine, or surgery.

Normal free T4 (FT4) in adults is typically about 0.7–1.9 ng/dL (9–24 pmol/L), though many laboratories report a reference range roughly 0.8–1.8 ng/dL (10–23 pmol/L). Exact cutoffs vary by assay, age and pregnancy status. FT4 should be interpreted alongside TSH and clinical findings; consult your clinician for personalized interpretation.

Abnormal T4 levels indicate thyroid dysfunction and can be serious if untreated. Elevated T4 (hyperthyroidism) may cause palpitations, weight loss, anxiety and, rarely, life‑threatening thyrotoxic crisis or heart complications. Low T4 (hypothyroidism) causes fatigue, cold intolerance, weight gain and, in severe cases, myxedema coma. Both are diagnosed by blood tests and are usually manageable with medication and monitoring—see a clinician.

Thyroid problems can affect weight: underactive thyroid (hypothyroidism) often causes modest weight gain from a slower metabolism and fluid retention, while overactive thyroid (hyperthyroidism) typically causes weight loss. Treatment usually reverses some change, but thyroid disease alone rarely causes extreme weight gain. If you have unexplained weight changes, ask your doctor to check TSH and free thyroid hormones.