The thyroid gland, a small butterfly-shaped organ located at the base of your neck, plays a crucial role in regulating your metabolism, energy levels, and overall body function. It produces hormones, primarily T3 (triiodothyronine) and T4 (thyroxine), which influence nearly every cell in your body. Thyroid function tests are essential for diagnosing and monitoring conditions like hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid).
Thyroid-Stimulating Hormone (TSH) is often the first and most important test for assessing thyroid function. TSH is produced by the pituitary gland in your brain and acts as a messenger, telling your thyroid gland how much T3 and T4 to produce. If your thyroid gland is underactive (hypothyroidism), your pituitary gland will produce more TSH to try and stimulate it, resulting in a high TSH level. Conversely, if your thyroid gland is overactive (hyperthyroidism), your pituitary gland will produce less TSH, leading to a low TSH level.
T3 (Triiodothyronine) and T4 (Thyroxine) are the main hormones produced by the thyroid gland. These tests measure the actual levels of these hormones circulating in your blood. Often, both total T3/T4 and free T3/T4 are measured. Free T3 and T4 are the unbound, active forms of the hormones that are available to your cells. Abnormal levels of T3 and T4, especially when combined with abnormal TSH levels, can confirm a diagnosis of hypothyroidism or hyperthyroidism.