The thyroid is a butterfly-shaped gland that lies in front of the
windpipe (trachea), just below the voice box (larynx). The thyroid gland uses
iodine from food to make two thyroid hormones: thyroxine (T4) and
triiodothyronine (T3).
The thyroid gland stores these thyroid hormones and releases them as
they are needed. Thyroid hormones regulate the way the body uses energy. A lack
of thyroid hormones (hypothyroidism) or an excess of thyroid hormones
(hyperthyroidism) affects many body systems and can cause a wide range of
symptoms, including changes in weight, heartbeat, body temperature, digestion,
and muscle function.
An overactive thyroid gland (hyperthyroidism) may affect how
electrical signals are generated and passed on in the heart, increasing the
risk of
atrial fibrillation.