The thyroid gland produces, stores, and secretes thyroid hormone that increases the body’s basal metabolic rate and calcitonin that reduces blood calcium levels. The thyroid is a unique gland because it partially synthesizes and stores its hormone product in the lumens of the thyroid follicles. The thyroid follicles are lined by a simple cuboidal epithelium containing thyrocytes (follicular cells or principle cells, produce thyroid hormone) and parafollicular cells (produce calcitonin). The follicle lumens are filled with an eosinophilic material called colloid where thyroid hormone is synthesized and stored.

• Examine the images below and these two sections of thyroid (sample 1, sample 2). The thyroid is surrounded by a thin connective tissue capsule and is divided into irregular lobules by septa that extend from the capsule. Identify thyroid follicles, colloid, thyrocytes, and parafollicular cells.

Clinical note: Increased height of thyroid follicular cells and infoldings of the follicle wall are indications of hyperthyroidism or Grave’s disease, a disorder characterized by restlessness, sleeplessness, tremor, and exophthalmos. Treatment is subtotal thyroidectomy or use of radioactive iodine to destroy or inactivate the thyroid follicles.
 

Parathyroids are next.