An underactive thyroid gland, called hypothyroidism in the medical literature, is a relatively common endocrine disorder, affecting an estimated 10 million people in the United States alone. The onset of hypothyroidism is occasionally rapid, as for example after radiation or surgery to treat a goiter, but more often occurs insidiously over the course of months to years. Although an underactive thyroid occurs more often in females than males, the signs and symptoms are often quite similar. These include fatigue, cold intolerance, involuntary weight gain, a puffy appearance termed myxedema; constipation; thin, dry hair, and other signs of a generally slow metabolism such as low heart rate (bradycardia), low blood pressure (hypotension), loss of libido, lethargy, and sometimes, major depression.
Blood work typically reveals a high level of TSH (Thyroid Stimulating Hormone) as well as hyperlipidemia, in particular a high level of LDL cholesterol. The reason TSH levels rise is straightforward. When the thyroid gland produces little or no thyroid hormone, the pituitary senses this deficiency and releases more TSH, in an ultimately futile attempt to jump start the thyroid gland. Physiologists are uncertain about the reason people with hypothyroidism often have high cholesterol. One possibility is that the liver turns over cholesterol and cholesterol containing molecules like bile at a slower rate, leading to the accumulation of dietary lipids in the blood circulation.
Medical causes of hypothyroidism:
– Autoimmune thyroiditis, a.k.a. Hashimoto’s disease, is far and away the most common underlying cause. For unknown reasons, white blood cells called B and T lymphocytes launch a two pronged assault on the thyroid gland. B cells produce antibodies targeting receptors on the cells lining the thyroid’s follicles. Meanwhile, T cells attack and kill these cells directly. After 90% or so of the follicular cells have been destroyed, the symptoms of hypothyroidism ensue.
– Other forms of thyroiditis (e.g. DeQuervain’s subacute thyroiditis) tend to be self limited but can produce transient hypothyroidism.
– Exposure to large amounts of radioactive iodine can obliterate the thyroid gland. This scenario occurred in the aftermath of the atomic bombs dropped on Japan in 1945 and the Chernobyl meltdown in 1986.
– Iodine deficiency. This condition was formerly common in mountainous areas far from the ocean, such as Switzerland, and the interior regions of continents including the Midwestern U.S. By the early 20th century, however, many countries, including the U.S., began adding iodine to most commercially available salt. Today, iodine deficiency remains a problem in remote parts of the Himalayas, Africa, and the Indonesian archipelago.
– Any condition affecting TSH release from the pituitary can, at least in theory, cause hypothyroidism. These conditions include hypothalamic/pituitary tumors, sarcoidosis, and postpartum panhypopituitarism, also known as Sheehan’s syndrome. Compared to the other causes of hypothyroidism, however, those stemming from TSH deficiency are relatively rare.