Final answer:
Advanced hypothyroidism in adulthood, known as myxedema, is commonly a result of Hashimoto's thyroiditis, an autoimmune condition. It's crucial to differentiate this from hyperthyroidism caused by Graves' disease, which has different symptoms and implications.
Step-by-step explanation:
Advanced hypothyroidism in adulthood is commonly caused by myxedema. Myxedema is a severe form of hypothyroidism characterized by thick, dry, and waxy skin, dull mental ability, and high sensitivity to cold. It's important to note that myxedema can be a manifestation of severely advanced hypothyroidism rather than its cause; this condition results from prolonged, untreated hypothyroidism. The most common cause of hypothyroidism in places where iodine deficiency is not a problem, such as the United States, is Hashimoto's thyroiditis, an autoimmune condition where the immune system attacks and destroys the thyroid gland.
Hashimoto's thyroiditis often occurs alongside other autoimmune disorders and is marked by symptoms that include goiter, cold intolerance, muscle weakness, joint pain, depression, and memory loss. Also, Graves' disease is associated with hyperthyroidism, not hypothyroidism, and is characterized by overactive thyroid symptoms such as heat intolerance and rapid heart rate. It is important to distinguish between these thyroid disorders as their treatments and implications are different.