Interactions of halogenated industrial chemicals with transthyretin and effects on thyroid hormone levels in vivo

Halogenated organic chemicals are compounds that have one or more chlorine or bromine atoms. Examples of such chemicals include polychlorinated biphenyls (PCBs), polybrominated biphenyls (PBBs), tetrachloro-p-dioxin (TCDD, or simply dioxin), dichlorophenoxy butanoic acid (2,4-DB), and dichlorodiphenyltrichloroethane (DDT). These substances are made in industry and used as insecticides. Previous studies have indicated that these chemicals may cause blood levels of thyroid hormones to be reduced and cause the thyroid gland to become enlarged (hypertrophy). It is not known how these chemicals reduce the levels of thyroid hormones. Transthyretin (TTR) is a protein that binds to and transports thyroxine (a thyroid hormone) in the blood. In order for thyroxine to perform its normal functions it must bind to and be carried in the blood by TTR. It has been hypothesized that PCBs may reduce blood levels of thyroxine by binding to TTR so that the thyroxine cannot bind to it. To investigate this hypothesis, 65 different halogenated organic chemicals commonly used in industry were tested for their ability to prevent thyroxin from binding to its carrier protein, TTR. The results of this study showed that many of the compounds tested were able to bind to TTR and therefore may be able to prevent thyroxine from binding to TTR in the blood. Examples of these chemicals include chlorophenols, dinoseb, bromoxynil and dichlorophenoxybutyric acid. It is suggested that thyroid function be measured in workers exposed to halogenated organic chemicals. (Consumer Summary produced by Reliance Medical Information, Inc.)