The Collaborative International Dictionary of English v.0.48:

Thyroid \Thy"roid\, a. [Gr. qyreoeidh`s shield-shaped; qyreo`s a
   large, oblong shield (from qy`ra a door) + e'i^dos form: cf.
   F. thyro["i]de, thyr['e]o["i]de.]
   1. Shaped like an oblong shield; shield-shaped; as, the
      thyroid cartilage.
      [1913 Webster]

   2. (Anat.) Of or pertaining to the thyroid body, thyroid
      cartilage, or thyroid artery; thyroideal.
      [1913 Webster]

   Thyroid cartilage. See under Larynx.

   Thyroid body, or Thyroid gland (Anat.), an endocrine
      gland having two lobes, located in the floor of the mouth
      or the region of the larynx, which secretes two hormones
      (thyroxine and triiodothyronine) that regulate the
      rates of growth and metabloism. In man and most mammals it
      is a highly vascular organ, partly surrounding the base of
      the larynx and the upper part of the trachea. Cases of
      hypothyroidism, where the gland is insufficiently active,
      can be treated by administration of thyroxine or a
      combination of thyroxine and triiodothyronine.

   Thyroid dislocation (Surg.), dislocation of the thigh bone
      into the thyroid foramen.

   Thyroid foramen, the obturator foramen.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thyroxine
    n 1: hormone produced by the thyroid glands to regulate
         metabolism by controlling the rate of oxidation in cells;
         "thyroxine is 65% iodine" [syn: thyroxine, thyroxin,
         tetraiodothyronine, T]