Читать книгу The Healthy Thyroid: What you can do to prevent and alleviate thyroid imbalance - Patsy Westcott - Страница 32

Most of the thyroid hormone in the body is carried around the bloodstream attached to special transport proteins, especially thyroid-binding globulin (TBG). Once the bound thyroid hormone reaches its destination, it is released from the protein binding so that T₄ can be converted to T₃ and ready for use by the cells. A tiny amount – around 0.03 per cent of T₄ and 0.3 per cent of T₃ – remains unattached to float freely about in the blood. Although only a small quantity, free-floating T₃ does not have to be released from any binding and so is immediately available for use by the cells.

Certain conditions, such as taking the Pill, can raise the levels of protein in the blood and, in the past, thyroid tests which measured total levels of T₄ and T₃ were not always accurate because of some confusion in interpreting results. Today’s blood tests measure levels of both free-floating T₄ and bound T₄ as well as thyroid-stimulating hormone, which provide a much more accurate indication of thyroid function.