Hypothyroidism is a common endocrine disorder in which the thyroid gland does not produce enough thyroid hormone (primary hypothyroidism), or due to inadequate production of hypothalamic thyrotropin-releasing hormone (secondary hypothyroidism). The most common causes of acquired hypothyroidism are Hashimoto thyroiditis, postsurgical or postablative hypothyroidism, drug-induced, and postpartum thyroiditis. Hashimoto thyroiditis is the most common cause of hypothyroidism in adults and can be associated with other autoimmune processes, such as systemic lupus, rheumatoid arthritis, primary adrenal insufficiency, type 1 diabetes mellitus, or Sjogren syndrome.
Thyroid hormone is required for normal functioning of numerous tissues in the body. A normal functioning thyroid gland predominantly secretes thyroxine (T4), which is converted into tri-iodothyronine (T3). The thyroid gland is the only source of thyroid hormone in the body; the process requires iodine and the amino acid thyrosine. Iodine in the bloodstream is taken up by the gland and incorporated into thyroglobulin molecules. The process is controlled by the thyroid-stimulating hormone (TSH), which is secreted by the pituitary. Inadequate levels of iodine, or TSH, can result in decreased production of thyroid hormones. The hypothalamic–pituitary–thyroid axis plays a key role in maintaining thyroid hormone levels within normal limits. Production of TSH by the anterior pituitary gland is stimulated in turn by thyrotropin-releasing hormone (TRH), released from the hypothalamus. Production of TSH and TRH is decreased by thyroxine by a negative feedback process. Not enough TRH, which is uncommon, can lead to inadequate levels of TSH and thereby inadequate thyroid hormone production.
Manifestations of hypothyroidism depend on pathogenesis, duration, severity, and the age of the patient. The onset is usually gradual, taking several years for notable symptoms and signs to develop. However, it may be abrupt, taking only a few months to develop. Abrupt onset of hypothyroidism occurs most commonly after radioiodine treatment for hyperthyroidism. In some cases, hypothyroidism is very mild and difficult to recognize because it causes few symptoms. In other cases, hypothyroidism can be very severe, especially if it is allowed to progress untreated for months or years. Some common symptoms of hypothyroidism are: lethargy, weight gain, cold intolerance, bradycardia, depressed myocardial contractility, cool dry skin, coagulation abnormalities, constipation, dry thinning hair, depression, decrease sweating, slowed joint and muscle pain, heavy or irregular menstrual periods, and impaired fertility. Despite these symptoms, hypothyroidism develops slowly and as a result many people may not notice symptoms of the disease.
Myxedema coma is the most serious form of hypothyroidism. This is a rare but life-threatening state of extreme hypothyroidism that is often precipitated by other conditions such as surgery, pituitary tumor, postpartum pituitary necrosis, drugs, trauma, and infectious state. The illness is characterized by very low body temperature (without shivering), confusion, a slow heart rate, congestive heart failure, and reduced breathing effort. Mortality is 100% if not treated.
Hypothyroidism leads to a variety of multisystemic effects: