Arises from the brachiocephalic trunk and ascends in front of the trachea to the lower part of the thyroid gland, which it supplies. It is only present in approximately 3-10% of the population. It varies greatly in size, and appears to compensate for deficiency or absence of one of the other thyroid vessels. Occasionally, it arises from the aorta, the right common carotid, the subclavian or the internal thoracic artery. (wikipedia.org)