The thyroid is a butterfly-shaped gland in the neck, just below the Adam's apple, that releases thyroid hormones T3 (triiodothyronine) and T4 (thyroxine). The thyroid regulates metabolism and the function of every cell in the body, and hypothyroidism is defined by inadequate levels of thyroid hormones. Although all hypothyroidism symptoms are related to a slowed metabolism, the symptoms are inconspicuous and vary depending on age of onset.
Any defect in the hypothalamic-pituitary-thyroid axis or in the structure of the thyroid gland can cause inadequate levels of thyroid hormones. Causes of hypothyroidism vary widely and include radiation injury, surgical ablation, thyroid hormone synthesis dysfunction, pituitary dysfunction, hypothalamic lesions, iodine deficiency, viral infections, medications such as lithium, inborn errors of biosynthesis and autoimmune disease. Hashimoto's thyroiditis, an autoimmune disease, is the most common cause of hypothyroidism in the United States and has a genetic component.
Symptoms of hypothyroidism revolve around slowed physical and mental activity. Patients complain of fatigue, weight gain, depression, difficulty concentrating, cold intolerance, bradycardia, high cholesterol, reactive hypoglycaemia, depression, constipation, migraines, muscle weakness, muscle cramps, abnormal ovulation and menstrual cycles, dry skin and hair, brittle nails, memory loss, hair loss and facial puffiness. An enlarged thyroid gland, called goitre, is a prominent symptom in autoimmune-mediated hypothyroidism.
Hypothyroidism in infancy or early childhood is called cretinism. This disorder occurs most commonly in areas with dietary iodine deficiency but can also occur due to inborn errors of metabolism. Symptoms include impaired development of the skeletal and neurological systems, severe mental retardation, short stature, coarse facial features, thick tongue and umbilical hernia. The severity is directly related to the time at which the deficiency occurred in utero or the age of onset in the child.
Myxedema refers to hypothyroidism that develops in an older child or young adult. Older children typically show symptoms between cretinism and adult-onset hypothyroidism. Symptoms include cold intolerance, weight gain, fatigue, mental sluggishness, shortness of breath due to decreased cardiac output, decreased exercise capacity, constipation, oedema and facial puffiness.
Myxedema coma is a potentially lethal complication, the most extreme presentation of hypothyroidism. This occurs when the body's attempts to compensate for inadequate thyroid hormones are overwhelmed--such as by infection, surgery and some medications. A diagnosis of myxedema coma does not necessarily require the person to be comatose. The patient typically presents with decreased mental function and multiple organ dysfunction. Treatment involves IV thyroid hormones and hydrocortisone.
A physician can diagnose hypothyroidism based on blood tests measuring the amount of thyroid hormone in the blood. Additional studies are often necessary to determine the cause of hypothyroidism and the proper treatment. Many forms of hypothyroidism are easily treated with hormone supplementation and close monitoring.