Thyroid: Anatomy of the Thyroid Gland and Thyroid Function

The neck houses some very important structures like the trachea, esophagus and the great vessels. Located among them is a structure very important for the well-being of the human body – the thyroid gland. This gland, a part of the endocrine system, produces thyroid hormones that are vital for growth and normal functioning of nearly every system in the body.

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Thyroid Anatomy – Structure of the Thyroid Gland

The thyroid gland is a butterfly-shaped (or H-shaped) structure located in the midline in the front of the neck, covered by flat muscles called the strap muscles. It has two lobes, the right and left lobes, shaped like the wings of a butterfly. The two lobes meet in the midline and are connected to each other by a small horizontal bar called the isthmus.

Sometimes, a pyramidal lobe may be seen arising from the isthmus, extending upwards. This lobe reflects the development of the thyroid from the primitive pharynx in the embryo. The thyroid gland develops from tissue in the region of the base of the tongue and gradually descends into the neck as the thyroglossal duct of which the pyramidal lobe is a remnant.

Weighing around 15 to 20 gm in adults, this is one of the largest endocrine glands and is larger in women than in men. It increases in size during pregnancy to meet the increased metabolic needs of the body. The back of the thyroid gland is attached to the cartilages of the larynx and upper rings of the trachea by a ligament called the Berry’s ligament. This attachment causes the gland to move up and down with the larynx when a person swallows. This is one of the features used in the diagnosis of thyroid swellings as they move during swallowing unlike other neck swellings.

Thyroid Hormones are Secreted by the Thyroid Gland

The thyroid gland has a thick capsule covering it, with lot of blood vessels. Inside, the gland is made up of a large number of fluid-filled sacs or vesicles called follicles, with delicate strands of connective tissue and small blood vessels running in between. These follicles are the structural units of the thyroid gland and are actively involved in the synthesis of thyroid hormones.

Each follicle is lined by cuboidal epithelial cells. These cells secrete a fluid called colloid that fills up the follicle. Colloid contains the glycoprotein thyroglobulin which is a precursor of the thyroid hormones, thyroxine and triiodothyronine, also called T4 and T3. Thyroid hormones are stored in the colloid, ready for release when required. Thyroid cells have the capacity to trap iodine that is required for the synthesis of thyroid hormones.

In between the follicles, amidst the connective tissue, some pale cells called C cells are strewn around. These cells secrete the hormone calcitonin required for calcium metabolism.

The thyroid gland is an endocrine gland, so it doesn’t have any ducts. The hormones it synthesizes are directly secreted into surrounding blood vessels and they circulate in the bloodstream to reach various organs for their effects.

Thyroid Function and Thyroid Problems

Thyroxine and triiodothyronine are the main hormones secreted by the thyroid gland. Both these hormones have important functions in regulating or rather stimulating body metabolism. The synthesis of these hormones is influenced by the action of one of the pituitary hormones called the thyroid stimulating hormone (TSH). Thyroid hormones specifically affect growth and body mechanisms like carbohydrate and fat metabolism and have stimulatory effects on the cardiovascular, respiratory and central nervous systems among others.

Conditions like the Graves’ disease cause remarkably increased thyroid function, a state called hyperthyroidism, resulting in symptoms like exophthalmos or protruding eyeballs, increased metabolism, weight loss and fatigue. On the other hand, there are several conditions like goiters that cause hypothyroidism or decreased thyroid function with symptoms like weight gain, myxedema and thyroid swelling. Hypothyroidism during fetal life or infancy results in a condition called cretinism.

Sources

Romanes, G.J. “Cunningham’s Manual of Practical Anatomy”

Guyton and Hall, “Textbook of Medical Physiology”