Figure 10.

Representative environmental chemicals with structures similar to that of thyroid hormone. A) Thyroid hormone (T₄) is metabolized by a series of enzymes to form more a potent (outer ring deiodination by Dio1 and Dio2) or less potent (inner ring deiodination by Dio3) hormone. These metabolic events are important steps in the control of thyroid hormone action in tissues and take place in specific cells as described in the text. It is important to recognize that some environmental chemicals may interfere with specific enzymes, but little is known of this pathway. Some of these chemicals are: B, PCBs; C, triclosan; D, PBDEs; and E, BPA. [Modified from M. E. Gilbert et al: Developmental thyroid hormone disruption: prevalence, environmental contaminants and neurodevelopmental consequences. Neurotoxicology. 2012;33:842–852 (1322), with permission. © Elsevier.]