ABSTRACT
Thyroxine and triiodothyronine are produced by iodination of mono- and diiodotyrosine (MIT, DIT) within the matrix of the thyroglobulin molecule, followed by coupling to form the finished hormones. The disposal and metabolic clearance rate of DIT has been measured, and is unchanged by administration of KI or methimazole. In normal man approximately 120 1 of blood are cleared per 24 hours. Because the clearance rate is only moderately increased in thyrotoxicosis it appears that there is clearly no dehalogenase deficiency in that disorder. MIT and DIT are deiodinated by an enzyme that is found in liver, kidney, thyroid, and in lesser amounts in many other organs. It was first characterized by Roche et al. in 1952 in homogenates of sheep, dog, and rabbit tissue. They found that deiodination of DIT yields MIT as an intermediate product, and that the iodide that is released within the thyroid may be recycled for hormone synthesis.
