Hyperthyroid Disorders

Lesson 1: What is Hyperthyroidism?

The Thyroid Gland

Before starting this section, students are advised to read pages 43-53 in chapter 3, Engine and Fuel: The Thyroid and its Hormones in the book Graves’ Disease, A Practical Guide by Elaine A. Moore. This resource provides a detailed description of the thyroid gland and the production of thyroid hormone.

The thyroid gland is the largest of the endocrine glands. The thyroid normally weighs less than an ounce or approximately 15 to 20 grams, and it has the same consistency as muscle tissue. The thyroid is situated directly below the larynx or Adam’s apple in the anterior or front of the neck. The thyroid gland, which is supported by bony cartilage to keep it upright, is attached to the trachea and suspended by the larynx.

A thin reddish-brown fibrous capsule covers the entire thyroid gland. This capsule contains finger-like projections known as septa. The thyroid is composed of two butterfly-shaped lobes that lie on each side of the trachea just above the breastbone.

The right and left lobes of the thyroid gland are connected by a narrow two-to-six mm band of tissue known as the isthmus. Some people also have an additional smaller central lobe situated between the two major lobes. This central thyroid lobe is called the pyramidal lobe.

Viewed through a microscope, the thyroid gland is composed of follicles or sacs of varying size. Each follicle is lined by a layer of epithelial cells, and each follicle contains a central storage area or lumen filled with colloid protein. The major protein found in the thyroid lumen is thyroglobulin. This protein is needed to carry molecules of thyroid hormone throughout the blood circulation.

Thyroid follicular cells or follicles are the most important of the thyroid cells. They trap iodine, which is used to produce thyroid hormone. The amount of iodine present in the blood circulation is called a rate-determining factor. That is, the amount of available iodine directly influences the amount or rate of thyroid hormone that’s produced.

The lobes of the thyroid gland are seen microscopically as containing many lobules, and each lobule contains 20 to 40 spherical follicles, which are also called thyroid follicular cells. Blood vessels, nerve fibers and blood cells hold the follicles together.

The thyroid gland also contains parafollicular or C cells that secrete a hormone known as calcitonin. Enlarged follicular cells often develop changes that alter their appearance. Some of these altered follicular cells with characteristic changes include oxyphil cells, Askanazy cells and Hurthle cells.

The thyroid gland primarily produces thyroid hormones and a smaller amount of the hormone calcitonin, which regulates calcium and phosphorus levels. Thyroid hormones influence the function of all of the body’s organs. For instance, thyroid hormone is essential for the proper functioning of the heart, brain, liver, kidneys and skin.

The Parathyroid Glands: The human body has four or more parathyroid glands that are situated on the rear or posterior thyroid lobe tips. The parathyroid glands secrete parathyroid hormone, a hormone that regulates calcium and phosphorus levels. Because of their proximity to the thyroid gland, the parathyroid glands can be damaged during thyroid surgery.

The Thyroid Gland in Hyperthyroidism: The appearance of the thyroid gland changes in hyperthyroidism. Thyroid follicles increase in size to keep up with the increased production of thyroid hormone. This increase in follicular cell size is called hypertrophy. The increased number of follicular cells seen in hyperthyroidism is called hyperplasia. Imaging tests of hyperthyroid glands frequently note that the cells appear hypertrophic with hyperplasia.

In hyperthyroidism, the gland is composed of crowded clusters of tall cells that form tufts or crowns. These tufts push into the lumen’s colloid causing holes or vacuoles. The surface of the thyroid gland usually remains smooth in hyperthyroidism caused by Graves’ disease although it may be lobulated. In toxic multinodular goiter, the surface may be studded with small nodules.

Goiter or enlargement of the gland may be visible in patients with hyperthyroidism. The gland can enlarge to six times its normal size, that is, up to 100 grams. On average, the hyperthyroid gland weighs 45 grams. How pronounced the goiter appears visibly depends on the normal position of the gland.

Thyroid glands that are retrosternal and situated further back in the throat may not be evident although they may interfere with swallowing when enlarged. Goiter occurs as the cells enlarge in their efforts to trap more iodine and produce more thyroid hormone. If the thyroid gland is situated toward the front of the neck, even a slight enlargement may be visible.

Enlargement in hyperthyroidism caused by Graves’ disease is uniform or diffuse whereas in nodular goiter the enlargement may occur in areas of tissue where nodules are present. In hyperthyroidism, one lobe often shows more of an enlargement than the other.

Thrills and bruits are often apparent in hyperthyroidism. Thrill refers to the increased blood flow through the thyroid and may be felt when the gland is palpated. Bruit refers to the audible sound of this increased blood flow. Bruit may be noticeable on one side of the gland or on the entire gland. Both thrills and bruits tend to decrease in intensity as hyperthyroidism is corrected.

The Neck Check: The American Association of Clinical Endocrinologists has designed a neck check that patients can use to help tell if they have a visible goiter. A glass of water and a handheld mirror are needed. Take a drink of water and swallow. Examine the area of the neck just below the Adam’s apple and immediately above the collarbone while tipping your head back slightly. Don’t confuse the Adam’s apple with your thyroid gland. Watch for bulges or protrusions in the area of the thyroid gland as you swallow. See your doctor if any bulges or protrusions are observed.

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