What condition can result from untreated severe hyperthyroidism?

Untreated severe hyperthyroidism can lead to a grave condition known as thyroid storm, which is characterized by a rapid and extreme exacerbation of hyperthyroid symptoms. This condition is a medical emergency and can result in severe complications such as heart failure, hyperthermia, and altered mental status.

During thyroid storm, the excessive levels of thyroid hormones can cause an overactivity of multiple body systems, particularly the cardiovascular system. Symptoms may include a high fever, tachycardia, intense agitation, and significant metabolic disturbances. Because this situation can arise when hyperthyroidism is not effectively managed, it highlights the importance of consistent monitoring and treatment of thyroid conditions.

The other options listed do not arise directly from untreated severe hyperthyroidism. For example, hypothyroid myxedema is a condition linked to severe hypothyroidism, Hashimoto's thyroiditis is an autoimmune disorder leading to hypothyroidism rather than hyperthyroidism, and adrenal crisis relates to acute adrenal insufficiency rather than thyroid imbalances. Therefore, the association of thyroid storm with untreated severe hyperthyroidism is what makes it the correct answer in this context.