Thyroid Disease… Overview of thyroid function
The thyroid gland is located in the front of the neck attached to the lower part of the voicebox (or larynx) and to the upper part of the windpipe (or trachea). It has two sides or lobes. These lobes are connected by a narrow neck (or isthmus). Each lobe is about 4 cm long and 1 to 2 cm wide. The name “thyroid” comes from the Greek word which means “shield”.
The thyroid gland produces thyroid hormones. These are peptides containing iodine. The two most important hormones are thyroxine (T4) and triiodothyronine (T3). These hormones are essential for life and have many effects on body metabolism, growth, and development.
Iodine plays an important role in the function of the thyroid gland. It is the chief component of thyroid hormones, and is essential for their production. Iodine is obtained from the water we drink and the food we eat. In areas of the world where there is an iodine deficiency, iodine must be added to the salt or bread. The Great Lakes area of Canada and the U.S., the Swiss Alps and Tasmania are such areas. In Canada and the U.S., most of the salt is iodized, thus the iodine intake is more than adequate. Taking excess amounts of iodine in supplements or foods such as kelp can aggravate hyperthyroid disease.
Enlargement of the thyroid gland is called goitre. Goitre does not always indicate a disease, since thyroid enlargement can also be caused by physiological conditions such as puberty and pregnancy.
Hypothalamic – Pituitary – Thyroid Axis
The thyroid gland is influenced by hormones produced by two other organs:
- The pituitary gland, located at the base of the brain, produces thyroid stimulating hormone (TSH)
- The hypothalamus, a small part of the brain above the pituitary, produces thyrotropin releasing hormone (TRH).
Low levels of thyroid hormones in the blood are detected by the hypothalamus and the pituitary. TRH is released, stimulating the pituitary to release TSH. Increased levels of TSH, in turn, stimulate the thyroid to produce more thyroid hormone, thereby returning the level of thyroid hormone in the blood back to normal.
The three glands and the hormones they produce make up the “Hypothalamic – Pituitary – Thyroid axis.”
The main types of thyroid dysfunction are:
- Too much thyroid hormone production or hyperthyroidism.
- Too little thyroid hormone production or hypothyroidism.
The state of normal thyroid function is called euthyroidism. Thyroid dysfunction is common is common and affects 1-5% of the population. All thyroid disorders are much more common in women than in men. Because of the widespread use of iodized salt and bread, lack of iodine is no longer a cause of thyroid disease in Canada as it was some 50 years ago.
“Autoimmune disorders” of the thyroid gland are the most common cause of thyroid dysfunction. These autoimmune disorders are caused by abnormal proteins, (called antibodies), and the white blood cells which act together to stimulate or damage the thyroid gland. Graves’ disease (hyperthyroidism) and Hashimoto’s thyroiditis, are diseases of this type. Graves’ disease affects about 1 in 100 of the population, whereas Hashimoto’s thyroiditis is even more common.
Graves’ disease (hyperthyroidism) is due to a unique antibody called “thyroid stimulating antibody” which resembles TSH and so can bind to the thyroid and ‘trick’ it into producing excessive amounts of thyroid hormones and stimulate thyroid growth.
In Hashimoto’s thyroiditis, there are antibodies that bind to substances in the thyroid and cause an immune response. There is an accumulation of white blood cells and fluid (inflammation) in the thyroid gland. This leads to destruction of the thyroid cells and, eventually, thyroid failure (hypothyroidism). In the beginning, thyroid hormone production decreases. In response to lower thyroid hormones levels, TSH increases and goitre can develop. In the later stages, the goitre can disappear because of the progressive destruction of the thyroid. Despite the presence of inflammation this is not a painful condition.
Sometimes, thyroid enlargement is restricted to specific parts of the gland; the rest of the gland being normal. The most common cause of this is a cyst or nodule, which may be benign or malignant. Often there is more than one nodule, and occasionally nodules can replace most of the thyroid, which can be called a multinodular goitre. Genetic modifications of the proteins usually involved in thyroid growth and function can contribute to this phenomenon. A thyroid nodule can be a benign or cancerous tumour, a cyst, or a localized section or wide spread areas in the thyroid with excess thyroid tissue growth that is otherwise normal (hyperplastic). Thyroid nodules are very common, increase with age and most are not cancers, though assessment to establish the risk of cancer is necessary.
Updated in August 2018 by Deric Morrison, MD, FRCPC, Div. of Endocrinology, Dept. of Medicine, University of Western Ontario. Original text written by Irving B. Rosen, MD, FRCS(C), FACS, Professor of Surgery, University of Toronto, Department of Surgery, Mount Sinai Hospital; Consultant in Surgery, Princess Margaret Hospital, Ontario Cancer Institute and Paul G. Walfish CM, MD, FRCP(C), FACP, FRSM., Professor of Medicine, Pediatrics and Otolaryngology, University of Toronto; Senior Consultant, Endocrinology and Metabolism and Head and Neck Oncology Program, Mount Sinai Hospital.