Hypothyroidism refers to the low or under functioning of the thyroid gland, resulting in the deficiency of thyroid hormones being produced. This gland is located in the front of the neck, and is the hormones produced are active in the control of metabolism and growth in virtually every cell of the body, so a deficiency can affect all body functions.
When the thyroid gland is working properly, it uses the amino acid tyrosine and iodine to make the thyroid hormone called thyroxine or T4. The other important thyroid hormone is triiodothyronine or T3. T3 is actually the major active thyroid hormone, being much more active than T4. T4 is produced within the thyroid gland and is later converted to the active T3 outside the thyroid gland in peripheral tissues.
The thyroid secretes its hormones in response to a thyroid-stimulating hormone (TSH) which is produced from the pituitary gland in the brain. At times when the thyroid hormones are low, the pituitary will secrete more stimulating hormone, but for some reason the thyroid gland does not respond. This is called primary hypothyroidism, and accounts for approximately 20% of females and 5% of males with in the population with thyroid disease. When both pituitary and thyroid hormones are low, it is termed secondary hypothyroidism.
Iodine is needed by the thyroid gland to produce its hormones, and iodine deficiency can lead to hypothyroidism and to the development of an enlarged thyroid gland, commonly known as goiter. Goiters can develop in response to the stimulation from the pituitary gland hormone TSH, and the thyroid gland grows in response. In this situation the cells of the gland enlarge but the activity is not increased because of lack of iodine.
Some foods can interfere with thyroid function. These are called goitrogens because excess ingestion may contribute to an imbalance of thyroid function and promote goiter formation. Foods in the cabbage family such as cabbage, turnips, Swede’s, Brussels sprouts, broccoli, and cauliflower, soy bean foods, maize, sweet potatoes, lima beans, soy, and pearl millet are all considered goitrogens because they contain chemicals that interfere with thyroid function or the utilization of iodine.
Hashimoto's thyroiditis (also called chronic lymphocytic thyroiditis) is an autoimmune condition of the thyroid gland where white blood cells infiltrate the gland itself and cause glandular destruction. It is the most common cause of hypothyroidism in the United States.
A form of congenital hypothyroidism can occur, called cretinism. The term cretinism was originally applied to infants in areas of low iodide intake and goiter, and these children have developmental delays and mental retardation. Testing for congenital hypothyroidism should happen at birth. Babies born to women with Hashimoto’s thyroiditis have an increased risk of cretinism.
The toxins PCB and PBB have adverse effects on the thyroid, but it is not known how important a contributor it is to the incidence of thyroid disease in the general population. Preliminary studies have found an association between multiple chemical sensitivities and hypothyroidism. One study found a correlation between high blood levels of lead, a toxic heavy metal, and low thyroid hormone levels in people working in a brass foundry. Many of these people also complained of depression, fatigue, constipation, and poor memory (symptoms of hypothyroidism). Occupational exposure to polybrominated biphenyls and carbon disulfide has also been associated with decreased thyroid function.
A very excessive intake of iodine, often from sea vegetables, has been associated with higher incidence of thyroid diseases, including hypothyroidism and thyroid cancer. Excessive iodine intake can result in either hypothyroidism or hyperthyroidism (overactive thyroid). Sources of iodine include foods (iodized salt, milk, water, seaweed, ground beef), dietary supplements (multiple vitamin-mineral formulas, seaweed extracts), drugs (potassium iodide, amiodarone, topical antiseptics), and iodine-containing solutions used in certain laboratory tests.
The clinical picture of hypothyroidism can range from minor irritating symptoms to the severe and life threatening. Skin changes are almost always seen, dry skin being the most common. Myxedema is also considered a skin sign; it is a particular kind of edema that can occur in the face with puffy eyelids and an enlarged tongue. It also appears on the hands and feet. There may be bruising due to poor healing and poor skin tone. Frequently a thinning of the eyebrows occurs. The face will be pale, and the voice may be hoarse. Gastrointestinal symptoms are common, especially constipation. Myxedema can also affect the heart and nervous system. The heart becomes enlarged and the nervous system may show a general slowing of the mental process, which can develop into depression or agitation and is term ‘myxedema madness’.
Hashimoto’s thyroiditis has a slightly different clinical picture, in that initially the symptoms may appear to be related to over activity of the thyroid gland. There may be a painless enlargement of the thyroid, but it may shrink later in the course of the condition. Hashimoto’s disease is also associated with joint pain, pernicious anemia and other forms of autoimmune diseases such as rheumatoid arthritis.