The Thyroid

The thryroid gland is located just inferior to the larynx, anterior to the trachea.  It has two lobes which are connected by the isthmus.  Under the microscope you will see a pattern of "rings" made of simple cubiodal epithelial tissue.  These epithelial cells are called follicle cells.  Inside of each "ring" is the colloid filled follicle.

The thyroid produces these hormones:

Triiodothyronine (T₃) and Thyroxine(T₄) :  Together these hormones are referred to as Thyroid Hormone.  Their function is to stimulate growth and influence tissue differentiation.  Thyroid hormone also stimulates the metabolic rate of all cells.  Dietary iodine is necessary for the production of thyroid hormone.

Calcitonin:  When blood calcium levels are high, calcitonin inhibits osteoclast activity and increases calcium deposition in the bone.

The Parathyroid Glands

The parathyroid glands are attached to the posterior surface of the thyroid gland.  These glands secrete:

Parathyroid hormone (PTH):  When blood levels of calcium are low, parathyroid hormone stimulates osteoclast activity which breaks down bone and releases calcium into the blood.  It also stimulates the kidneys to release calcitriol, which is the active form of Vitamin D.  Vitamin D is necessary for the body to absorb dietary calcium.