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Thyroid function tests are laboratory examinations used to evaluate how well your thyroid gland is working. The thyroid is a small butterfly-shaped gland located in the front of your neck that produces hormones essential for regulating your metabolism, energy levels, body temperature, heart rate, and many other vital functions. When the thyroid produces too much or too little hormone, it can cause various health problems that affect your overall wellbeing.

The most common thyroid function tests include TSH (thyroid-stimulating hormone), free T4 (thyroxine), and free T3 (triiodothyronine). TSH is produced by the pituitary gland in your brain and signals the thyroid to produce hormones. When thyroid hormone levels are low, TSH levels rise to stimulate more production. When thyroid hormone levels are high, TSH levels drop. Free T4 and free T3 are the actual hormones produced by the thyroid gland that are available in your bloodstream to perform their functions throughout your body. These tests are typically performed together to give a complete picture of thyroid function.

Your doctor may order thyroid function tests if you have symptoms suggesting thyroid problems, such as unexplained weight changes, fatigue, hair loss, changes in heart rate, mood changes, difficulty sleeping, sensitivity to temperature, or irregular menstrual periods. These tests are also used to monitor people already diagnosed with thyroid disorders who are receiving treatment, to screen newborns for congenital thyroid problems, and to evaluate thyroid nodules or an enlarged thyroid gland.

The tests require a simple blood sample, usually drawn from a vein in your arm. No special preparation is typically needed, though you should inform your healthcare provider about any medications or supplements you take, as some can affect test results. Results are usually available within a few days. Normal ranges can vary slightly between laboratories, but generally, abnormal results indicate either hypothyroidism (underactive thyroid) when TSH is high and T4/T3 are low, or hyperthyroidism (overactive thyroid) when TSH is low and T4/T3 are high. Your healthcare provider will interpret your results in the context of your symptoms and medical history to determine if treatment is necessary.