1) A gland located beneath the voice box (larynx) that produces thyroid hormone. The thyroid helps regulate growth and metabolism. Underactive thyroid is also called hypothyroidism. This is a condition in which the thyroid gland doesn't make enough thyroid hormone. Lack of thyroid hormone will slow down your metabolism and cause weight gain.
2) The endocrine gland normally situated in the lower part of the front of the neck; secretes, stores and releases the iodine dependent thyroid hormones thyroxine and triiodothyroine which play major endocrine roles in regulating metabolic rate; also secretes thyrocalcitonin.