Thyroid Gland Anatomy

• The thyroid is one of {the} largest endocrine glands in {the} body. This gland is found in {the} neck inferior to (below) {the} thyroid cartilage (sometimes referred to as {the} Adam's apple in men) and at approximately {the} same level as {the} cricoid cartilage. The thyroid controls how quickly {the} body burns energy, makes proteins, and controls how sensitive {the} body should be to other hormones.
• The thyroid participates in these processes by producing thyroid hormones, principally thyroxine (T₄) and triiodothyronine (T₃). These hormones regulate {the} rate of metabolism and affect {the} growth and rate of function of various other systems in {the} body. Iodine and tyrosine are used to form both T₃ and T₄. The thyroid also produces {the} hormone calcitonin, which plays a role in calcium homeostasis.
• The thyroid is controlled by {the} hypothalamus and pituitary. The gland gets its name from {the} Greek word for "shield", after {the} shape of {the} related thyroid cartilage. Hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid) are {the} most common problems of {the} thyroid gland.

• The thyroid gland is a butterfly-shaped organ and is composed of two cone-like lobes or wings: lobus dexter (right lobe) and lobus sinister (left lobe), connected with {the} isthmus. The organ is situated on {the} anterior side of {the} neck, lying against and around {the} larynx and trachea, reaching posteriorly {the} oesophagus and carotid sheath. It starts cranially at {the} oblique line on {the} thyroid cartilage (just below {the} laryngeal prominence or Adam's apple) and extends inferiorly to {the} fourth or fifth tracheal ring. It is difficult to demarcate {the} gland's upper and lower border with vertebral levels because it moves position in relation to these during swallowing.
• The thyroid gland is covered by a fibrous sheath, {the} capsula glandulae thyroidea, composed of an internal and external layer. The external layer is anteriorly continuous with {the} lamina pretrachealis fasciae cervicalis and posteriorolaterally continuous with {the} carotid sheath. The gland is covered anteriorly with infrahyoid muscles and laterally with {the} sternocleidomastoid muscle. Posteriorly, {the} gland is fixed to {the} cricoid and tracheal cartilage and cricopharyngeus muscle by a thickening of {the} fascia to form {the} posterior suspensory ligament of Berry^[In variable extent, Lalouette's Pyramid, a pyramidal extension of {the} thyroid lobe, is present at {the} most anterior side of {the} lobe. In this region {the} recurrent laryngeal nerve and {the} inferior thyroid artery pass next to or in {the} ligament and tubercle. Between {the} two layers of {the} capsule and on {the} posterior side of {the} lobes there are on each side two parathyroid glands.
• The thyroid isthmus is variable in presence and size, and can encompass a cranially extending pyramid lobe (lobus pyramidalis or processus pyramidalis), remnant of {the} thyroglossal duct. The thyroid is one of {the} larger endocrine glands, weighing 2-3 grams in neonates and 18-60 grams in adults, and is increased in pregnancy^.
• The thyroid is supplied with arterial blood from {the} superior thyroid artery, a branch of {the} external carotid artery, and {the} inferior thyroid artery, a branch of {the} thyrocervical trunk, and may be by {the} thyroid ima artery, branching directly from {the} aortic arch. The venous blood is drained via superior thyroid veins, draining in {the} internal jugular vein, and via inferior thyroid veins, draining via {the} plexus thyroideus impar in {the} left brachiocephalic vein. Lymphatic drainage passes often {the} lateral deep cervical lymph nodes and {the} pre- and parathracheal lymph nodes. The gland is supplied by sympathetic nerve input from {the} superior cervical ganglion and {the} cervicothoracic ganglion of {the} sympathetic trunkand by parasympathetic nerve input from {the} superior laryngeal nerve and {the} recurrent laryngeal nerve.