adjective

1. designating or of a large ductless gland lying in front and on either side of the trachea and secreting the hormone thyroxine, which regulates body growth and metabolism: the malfunctioning or congenital absence of this gland can cause goiter, cretinism, etc.
2. designating or of the principal cartilage of the larynx, forming the Adam's apple

Etymology: ModL thyroides < Gr thyreoeidēs, shield-shaped < thyreos, large shield, door-shaped shield < thyra, door + -eidēs, -oid

noun

1. the thyroid gland
2. the thyroid cartilage
3. an artery, nerve, etc. in the region of the thyroid
4. a preparation of the thyroid gland of certain animals, used in treating goiter, myxedema, etc.
   also thyroid extract