We present a 21-yr-old female with a large hepatic vascular tumor and subclinical hypothyroidism. A high level of the thyroid hormone inactivating enzyme type 3 iodothyronine deiodinase (D3) was detected in her tumor, and the TSH of 26.2 mU/liter returned to normal after surgical resection of the mass. This indicates that the vascular tumor caused this adult’s hypothyroidism as has now been documented in nine infants with this syndrome. This first example of consumptive hypothyroidism in an adult indicates that the inactivation rate of thyroid hormone by D3 in a vascular tumor can stress the secretory capacity even of the TSH-stimulated normal adult thyroid gland.