Lipoprotein-thyroid hormone interactions.

A small but significant portion of the thyroid hormones that circulate in human plasma is associated with lipoproteins. Although the major lipoprotein carrier is HDL, the role of these interactions on T(4) entry into cells was tested first with LDL and human fibroblasts because of the well-characterized LDL receptors and the availability of cells with genetically absent receptors. It was shown that LDL enhanced the uptake of T(4) by cultured fibroblasts in which the LDL receptors were expressed. The T(4) binding sites on apolipoproteins B-100 and E, as well as apoA-I, have been partially characterized, and they exhibit considerable homology. A number of possible physiologic consequences of thyroid hormonelipoprotein interactions have been put forward as topics for further investigation.