What condition is characterized by exophthalmos, pretibial myxedema, and decreased TSH levels?

The condition characterized by exophthalmos, pretibial myxedema, and decreased TSH levels is indeed Graves' disease. This autoimmune disorder leads to hyperthyroidism due to stimulating antibodies that activate the thyroid gland, leading to excessive production of thyroid hormones.

Exophthalmos, or bulging eyes, occurs because the excess thyroid hormone increases the production of glycosaminoglycans in the orbital tissues, causing inflammation and swelling. Pretibial myxedema, which presents as a thickening of the skin over the shins, is another manifestation associated with the accumulation of glycosaminoglycans and is particularly seen in Graves' disease.

The decreased levels of TSH (thyroid-stimulating hormone) are a direct consequence of the high levels of circulating thyroid hormones (T3 and T4) due to the feedback inhibition on the pituitary gland. In hyperthyroid states, such as in Graves' disease, the body reduces TSH production to decrease the stimulation of the thyroid gland.

Overall, the combination of these signs and symptoms distinctly points to Graves' disease as the underlying condition.