Underactive Thyroid Gland

Underactive thyroid gland is also known as the hypothyroidism. It is a failure of the thyroid gland function, which manifests by decreased production of its hormones. To understand the issue, it is good know some information about the activity of the thyroid gland. This organ is located in the neck in front of the trachea and consists of two lobes connected in the middle by a narrow neck. Thyroid cells are capable of capturing iodine from the blood and use it to produce thyroid hormones known as thyroxine (T4) and triiodothyronine (T3). These hormones have a broad spectrum of effects on the metabolism of nutrients and energy. The thyroid gland includes four parathyroid glands (two in each lobe), whose function affects the calcium and bone metabolism.


Thyroid gland


The majority of organs producing hormones are functionally connected with the brain and the thyroid gland is no exception. The production of thyroid hormones is controlled by two brain centers known as hypothalamus and pituitary gland.


The hypothalamus produces substance known as TRH acting on the pituitary gland, the pituitary gland releases additional substance (TSH) acting directly on the thyroid and causing increased production of T4 and T3 hormones. Quite logically, elevated serum concentration of T4 and T3 reduce the amount of produced TRH and TSH to adjust a feedback.


In fetuses and neonates, the hormones affect proper development of the brain. In adult humans, they are important for body metabolism, production of energy, maintenance of cholesterol levels, heart rate and both the physical and mental activity.


Low thyroid function may be caused by an autoimmune inflammation (known as Hashimoto's thyroiditis) of the gland causing the decrease of production of the thyroid hormones. Other cases of damage of the thyroid gland include infectious inflammations, irradiation, thyroid cancers, surgical removal of the thyroid gland, etc. The function of the thyroid gland is decreased when there is the iodine deficiency. Today, this situation is rare thanks to the iodization of salt, but in the past, it caused locations with endemic cretinism (see Symptoms). Rare causes of hypothyroidism may be also damages of the brain structures that stimulate the thyroid (injuries, strokes, brain tumors, etc.).


The symptoms of low thyroid function are related to the function of thyroid enzymes. When the thyroid deficiency occurs in a fetus or in a newborn (usually due to iodine deficiency), it causes a lifelong mental retardation known as the cretinism. In adult, the hormone deficiency does not cause the cretinism, but many other symptoms. The patients suffer from fatigue, passivity, certain increase of weight, constipation, high levels of cholesterol, dry skin, brittle nails, brittle hair, hair loss, cold feet and hands and muscle weakness. The victims have accelerated atherosclerosis and all its complications.


The reduced thyroid function also leads to accumulation of mucous substances (mucopolysaccharides) in the subcutaneous tissues and mucous membranes. This situation is known as myxedema. The tissue thickening may affect the vocal cords and cause changes of the voice. Long-term untreated hypothyroidism can cause unconsciousness and death.


The process may be associated with enlarged thyroid gland, which is referred to as the goiter. However, the goiter may be also present in normally functioning gland or in the overactive thyroid. Large goiter may compress the nearby located esophagus and trachea causing swallowing problems and shortness of breath.


The appropriate method of therapy is regular chronic administration of the thyroid hormones. In addition, when there is a treatable cause of the underactive thyroid, it is necessary to solve it.


Jiri Stefanek, MD  Author of texts: Jiri Stefanek, MD
 Contact: jiri.stefanek@seznam.cz
 Sources: basic text sources