8.3.2 Hyperthyroidism and Hypothyroidism
Hyperthyroidism occurs when your body makes too much of thyroid hormones，thyroxine（T4）and triiodothyronine（T3），and becomes overactive.The most common form of hyperthyroidism is Graves disease，also called diffuse toxic goiter.This is an autoimmune disorder in which antibodies stimulate an increased production of thyroid hormone.There is weight loss，irritability，hand tremor，and rapid heart rate（tachycardia）.A most distinctive sign is a bulging of the eyeballs，termed exophthalmos，caused by swelling of the tissues behind the eyes（Fig.8-3）.If you have hyperthyroidism，you may experience a fast heartbeat，increased appetite，anxiety，sensitivity to heat，or sudden weight loss.Medications，radioactive iodine，or surgery are treatment options of hyperthyroidism.If left untreated，hyperthyroidism can cause bone loss or an irregular heartbeat.
Fig.8-3 Exophthalmos
Hypothyroidism is a common condition where the thyroid doesn’t create and release enough thyroid hormone into your bloodstream.Because thyroid hormone affects the growth and function of many tissues，a deficiency of this hormone in infancy causes physical and mental retardation as well as other symptoms that together constitute congenital hypothyroidism，formerly called cretinism.In adults，thyroid deficiency causes myxedema，in which there is weight gain，lethargy，rough，dry skin，and facial swelling.Both of these conditions are easily treated with thyroid hormone.Most U.S.states now require testing of newborns for hypothyroidism.If not diagnosed at birth，hypothyroidism will lead to mental retardation within 6 months.The primary treatment for hypothyroidism is synthetic thyroxine taken in pill form.Some people who are still symptomatic on thyroxine replacement medications may be given T3 replacement hormones in addition to T4 replacement therapy.
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Task 4 Decide whether the following statements are true（T）or false（F）according to the text.(https://www.daowen.com)
1.Hyperthyroidism occurs when your body becomes inactive.（ ）
2.A patient with hypothyroidism may develop tachycardia，increased appetite，and sudden weight loss.（ ）
3.If a baby doesn’t make enough thyroid hormones，he（she）might experience serious problems such as mental disability and growth delays.（ ）
4.A most distinctive sign of Graves disease is eyeball protrusion due to swelling of the tissues behind the eyes.（ ）
5.Radioactive iodine uptake test can be performed to assess thyroid function.（ ）