Hypothyroidism is a clinical state resulting from decreased circulating levels of free thyroid hormone or from resistance to hormone action. Myxedema connotes severe hypothyroidism.
Hypothyroidism may be associated with Type II autoimmune polyglandular syndrome, which is associated with HLA-DR3, DR4. Secondary hypothyroidism frequently results from treatment for Graves disease, which may be familial.
Hypothyroidism is more commen in women (Female > Male, 5-10:1). It predominantly affects people over the age of 65.
Symptoms include: Onset may be insidious, subtle; Weakness, fatigue, lethargy; Cold intolerance; Decreased memory; Hearing impairment; Constipation; Muscle cramps; Arthralgias; Paresthesias; Modest weight gain (10 pounds [4.5 kg]); Decreased sweating; Menorrhagia; Depression; Hoarseness; Carpal tunnel syndrome.
Signs include: Dry, coarse skin; Dull facial expression; Coarsening or huskiness of voice; Periorbital puffiness; Swelling of hands and feet; Bradycardia; Hypothermia; Reduced systolic blood pressure; Increased diastolic blood pressure; Reduced body and scalp hair; Delayed relaxation of deep tendon reflexes; Macroglossia; Dilutional hyponatremia; Anemia (usually normochromic, normocytic); Enlarged heart on chest x-ray (often due to pericardial effusion).
Post-ablative (most common) hypothyroidism follows radioactive iodine therapy or thyroid surgery. Delayed hypothyroidism may develop in patients treated with thioamide drugs (propylthiouracil, methimazole) 4 to 25 years later.
Primary hypothyroidism may develop as a result of autoimmune thyroiditis, or be idiopathic. With goiter, hypothyroidism is most commonly due to autoimmune disease, such as Hashimoto's thyroiditis; or heritable biosynthetic defects, iodine deficiency (rare in the US), or drug induced (iodides, lithium, phenylbutazone, aminosalicylic acid).
Suprathyroid hypothyroidism, may be due to deficiency of thyrotropin-releasing hormone (TRH) from the hypothalamus or thyroid-stimulating hormone (TSH) from the pituitary.
Transient hypothyroidism may result from silent thyroiditis (most common in post partum period) and subacute granulomatous thyroiditis
Viral and bacterial infections can temporarily damage the thyroid gland, causing a short-term form of the condition. Hypothyroidism caused by infection usually does not result in permanent hypothyroidism.
Pregnancy, which requires an increased production of thyroid hormone, can cause hypothyroidism. About 2% of pregnant women in the United States develop hypothyroidism.
One of every twenty women develop thyroid inflammation within a few months after delivery. This is called postpartum thyroiditis, which usually goes away on its own after one to four months.
Hypothyroidism has been associated with irregular estrous cycles and changes in reproductive hormones, including: increased levels of progesterone and prolactin, and decreased estradiol.
Hypothyroidism may also be caused by iodine deficiency and excess. Iodine is a major component of thyroid hormones.
In autoimmune hypothyroidism, antibodies destroy thyroid gland cells preventing the gland from being able to release normal amounts of thyroid hormones. Hashimoto's thyroiditis results in a goitre, a swelling of the thyroid gland, that is visible as a lump on the neck. Autoimmune diseases include: AddisonÕs disease,
Risk factors for hypothyroidism include: increasing age and autoimmune diseases.
Thyroid hormones include: TSH, T4, and T3.
Thyroid peroxidase antibodies are not normally present, so a high value usually indicates autoimmune damage to the thyroid, including HashimotoÕs thyroiditis and GravesÕ disease.
Reverse T3 (rT3) may also be measured. T4 can form either T3 or reverse T3.
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