Pretibial myxedema is a skin condition that may develop in patients with Graves’ disease. It is characterized by thickening of a layer of tissue that lies directly beneath the surface of skin called the dermis. The word “pretibial” refers to the fact that this thickening usually involves the skin that covers the front of the tibia bone in the lower leg. Other terms used to identify this disorder include localized myxedema and thyroid dermopathy.
Pretibial myxedema usually becomes evident when raised lesions begin to appear on the skin that covers the front surfaces of the shins. These lesions are usually light in color, though they may darken over time. They are usually painless, though they may occasionally cause some itching. Hair follicles in these lesions may become very prominent, giving the overlying skin an appearance and texture that resembles an orange peel. Increased perspiration may also be noted. In most cases of pretibial myxedema, continued growth of lesions leads to widespread thickening of the skin that covers the front surfaces of the shins. In some cases limited growth of lesions may be associated with the emergence of circumscribed raised lesions called plaques or nodules. In rare cases, lesions may grow to the point where they block the flow of fluid through the lymphatic vessels. This may lead to severe swelling of the limbs. Although myxedema most commonly develops along the front surfaces of the shins, it may also involve the feet and toes. It is less likely to develop in other locations, though cases have been reported that have presented with thickening of skin along the arms, hands, fingers, back, ears, and nose.
While the underlying cause of pretibial myxedema is not completely understood, it has been suggested that it may represent an autoimmune disorder. There is some evidence that thyroid stimulating immunoglobulins produced in the setting of Graves’ disease may bind to structures present in the dermis. If this binding occurs, it may draw specific types of white blood cells into the skin. These white blood cells may release chemical compounds called cytokines that may promote the growth and division of cells in the dermis called fibroblasts. As fibroblasts grow and divide, they may produce and secrete chemical compounds called glycosaminoglycans. Deposits of glycosaminoglycans in the dermis may lead to progressive thickening of the skin identified as myxedema.
On the whole, pretibial myxedema represents a relatively rare complication of Graves’ disease. When it does develop, it is often associated with thyroid eye disease. It is more common among women. In rare cases, patients diagnosed with Graves’ disease may develop isolated thyroid eye disease and pretibial myxedema without any evidence of thyrotoxicosis or hyperthyroidism. This condition is called euthyroid Graves’ disease.