Along each lateral side of the trachea, the thyroid gland can be located in the neck, segregated into two distinct lobes of approximately 2 inches in length. Each lobe in intricately connected via the isthmus, a stub of tissue along the anterior segment of each lobe. Just under the larynx, the thyroid is the largest and most prominent of all the endocrine glands. Weighing about 20 grams, the thyroid gland relies on the two branches of the of the external carotid arteries along with the inferior thyroid branches which extend from the subclavian arteries for its high demand for vascular support. Venous drainage is supplied by the pair of superior and middle thyroid veins. These veins traverse the internal jugular veins and continue through the inferior thyroid vein to reach the brachiocephalic veins. Microscopic thyroid follicles comprise the majority of the structural thyroid gland. These appear to be sacs fashioned in a sphere and are empty or hollow on the inside. Follicular cells create the simple cuboidal epithelium that composes the sacs are lined with. The follicular cells are responsible for the creation of two specific hormones, the 2 main thyroid hormones. A fluid that is immensely rich in protein, known as colloid, can be found within the interior of the follicles.
Positioned in between each follicle are additional epithelial cells which are responsible for creating the hormones. The epithelial cells are called parafollicular cells and the hormones which they administer are calcitonin and thyrocalcitonin. Postganglionic neurons derived from the superior and middle cervical ganglia are in part responsible for the innervation of the thyroid gland. Additionally, second through the seventh thoracic segment of the spinal cord sends out branches to innervate the thyroid with preganglionic neurons.
The thyroid gland ultimately supplies the body with two main hormones, the thyroxine hormone and the triiodothyronine hormone. It also contributes the minor hormone, calcitonin. Both of the major hormones receive regulation from the adenohypophesis of the pituitary gland. The thyroid follicles are known to maintain these two main hormones and wait for impulses to release them into the body. They are responsible for controlling the body’s metabolic rate. Their main function involves increases the rate of protein synthesis and as well as regulates the release of energy derived directly from carbohydrate sources. In fetal and early development, these hormones specifically indicate the rate of maturation involving the nervous system. In a growing body, they regulate and stimulate the onset and continuation of sexual maturity, and they control the rate the human body grows.
The secondary hormone, calcitonin, works in cohesive unity with a parathyroid hormone (calcitonin is created by the parafollicular cells and is a polypeptide hormone) in order to regulate the levels and usage when referring to blood calcium levels. Without calcitonin, bone tissue would break down and the kidneys would not be able to expel calcium. These two actions work in unison to keep blood calcium levels lower.