The diagnosis of hypothyroidism is achieved by clinical and biochemical laboratory means. RAIU no longer plays a role in documenting hypothyroidism. The high level of iodine intake through diet and nutritional supplements in North America has reduced the normal range of RAIU such that there is now substantial overlap between the normal and hypothyroid ranges. Both congenital and acquired organification defects can result in situations in which the trapping function is preserved despite little or no production of organified iodine. Under the stimulation of elevated TSH, this can result in an elevated RAIU, often to very high values, in the face of profound hypothyroidism. Organification defects can be demonstrated by use of the perchlorate release test.
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