Thyroid

Lore & Background

The thyroid gland is composed of two connected lobes, left and right, joined by a thin band of tissue called the isthmus. It lies against the front of the larynx and trachea, with the isthmus extending from the second to third rings of the trachea. The gland is covered by a thin fibrous capsule and receives arterial blood from the superior and inferior thyroid arteries. Microscopically, its functional unit is the spherical thyroid follicle, lined with follicular cells and occasional parafollicular cells surrounding a lumen containing colloid.

Reader's Guide

The thyroid gland is significant for its role in producing hormones that regulate the body's metabolic rate, protein synthesis, and growth and development in children. Its secretion of calcitonin contributes to calcium homeostasis. Disorders of the thyroid, such as hyperthyroidism (often caused by Graves' disease) and hypothyroidism (commonly due to iodine deficiency or Hashimoto's thyroiditis), have major health impacts. In iodine-deficient regions, hypothyroidism is the leading cause of preventable intellectual disability in children. The gland's development begins in the embryo at 3–4 weeks gestational age, descending from the floor of the pharynx to its final position in the neck.

Did You Know?

• The thyroid gland weighs about 25 grams in adults and is usually larger in women than in men.
• A pyramidal lobe, a remnant of the thyroglossal duct, is present in 18.3% to 44.6% of people.
• The thyroid secretes three hormones: T3, T4, and calcitonin.
• The most common cause of hyperthyroidism is Graves' disease, an autoimmune disorder.