The iodothyronine deiodinases (DIO) are dimeric, membrane-bound enzymes that regulate the activity of thyroid hormone by removal of specific iodines from the precursor T4. There are three types of DIOs in humans; types I, II and III (D1, D2 and D3 respectively) which are proteins of about 250 residues that contain a selenocysteine at their active site. Signaling by thyroid hormone can change in individual tissues by this activation or inactivation process, even when serum concentrations of the hormone remain normal. Generally, cell types express just one type of DIO at any one time. The exception is the pituitary gland which expresses all three.