The terminology in this subject is a bit confused and has changed over time, so older studies will have the older terminology. The phrase ‘Impaired Sensitivity to Thyroid Hormone’ tends to be used to describe any form of disruption to thyroid hormone action. It used to be called ‘Reduced Sensitivity to Thyroid Hormone’ but ‘reduced’ was changed to ‘impaired’ because there is the possibility of increased sensitivity. There are three major forms of impaired sensitivity that are recognised. They feature abnormal blood tests results and have profound effects from birth (or conception).<\/p>\n\n\n\n
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This is where there is a defect in proteins that carry thyroid hormone across the cell membrane into the cells. There are a number of cell membrane transporters, the best known being the Monocarboxylate transporter 8 (MCT8). Mutations of the MCT8 transporter results in severe psychomotor defects. This is also known as ‘Allan-Herndon-Dudley syndrome’. MCT8 deficiency is a recessive X-linked defect that affects males, while females are carriers.<\/p>\n\n\n\n