"BPA binds to thyroid hormone receptors and estrogen receptors, antagonizing thyroid hormone receptor activation. BPA has also been shown to inhibit the regulation of most T3-dependent responsive genes and affect T3 signaling pathways. In an animal model, BPA exposure produced an endocrine profile similar to that observed in patients with T3 resistance syndrome.
PCBs are persistent organic pollutants that have paired phenol rings with different degrees of chlorination. Their structure is similar to thyroid hormone and can bind to interact with the thyroid hormone receptor acting as an agonist or antagonist to the thyroid hormone receptor. PCBs were widely used from 1930 to 1979, at which time they were banned in the US. They are lipophilic compounds that have a half-life of 7 years and have been found in high amounts in breast milk. Prenatal exposure to PCBs has been shown to be associated with decreased cognitive function in children, impairing executive function, verbal abilities, visual recognition and memory.
PBDEs have a structure similar to thyroid hormone and can displace T4 from serum thyroid-binding protein transthyretin (TTR). PBDEs are widely used as flame retardants and in plastics, foams and building materials, and have also been detected in foods (meat, fish, vegetables and dairy). PBDEs are detectable in 97% of US residents at levels 20 times higher than those seen in European residents. Increased PBDEs have also been associated with lower free T3; TSH declines 16% for every ten-fold increase in PBDEs, increasing the risk of subclinical hyperthyroidism two-fold.
Triclosan is an antibacterial/anti-fungal agent containing chlorinated organic molecules similar to PCBs, PBDEs and BPA. Triclosan suppresses serum thyroid hormone concentrations. According to NHANES 2003–2004, Triclosan has been found in 75% of urine samples and has also been found at detectable levels in breast milk.
Other environmental toxins which affect thyroid function and hormone homeostasis include organochlorine pesticides, which activate hepatic uridine diphosphate glucuronyltransferases (UDPTGs) and increases T4 metabolism, and perchlorate, which lowers both T3 and T4. Some degree of protection has been seen with iodine supplementation."