Ilonka A, TM Meerts, JJ van Zanden, EAC Luijks, I van Leeuwen-Bol, G Marsh, E Jakobsson , Å Bergman and A Brouwer. 2000. Potent Competitive Interactions of Some Brominated Flame Retardants and Related Compounds with Human Transthyretin in Vitro. Toxicological Sciences 56: 95-104.

Background on PBDEs

This research article elevates concern about flame retardants dramatically, especially given independent information about the ubiquity of flame retardant contamination and the potential impact of thyroid disruptions on brain development.

The bottom line is that some widespread flame retardants are more powerful than the natural hormones themselves at binding with transthyretin, a protein crucial to normal thyroid hormone function.These retardants are present. They are powerful. And their impact could be profound.

What did Ilonka et al. study?
The authors examined the ability of certain polybrominated diphenyl ethers (PBDEs), pentabromophenol (PBP), and tetrabromobisphenol A (TBBPA) to prevent natural human thyroid hormone, thyroxine (or T4), from binding with human transthryetin in test tube experiments.

Their logic was simple. Ilonka et al. were motivated by the observation that these flame retardants are similar in chemical form to natural human thyroid hormone T4. This raised concerns that if these compounds were present in people, then they might interfere with the natural binding of T4 to transthryetin, thus potentially leading to disruption of the thyroid control over brain development.

Test tube experiments like this, called competitive binding experiments, are well-accepted, indeed classic experiments used to assess the potency of interaction among natural and synthetic compounds. In the experiments, natural and synthetic compounds are mixed together in a way that allows the experimenters to measure whether the synthetic materials displaced the natural compounds from binding.

Synthetic compounds that compete effectively with the natural hormone are thought to be riskier than those that don't (all other things being equal), because if present they are more likely to interfere with natural hormone function. In most systems studied, e.g., estrogen (as opposed to thyroid) mimics, the synthetic compound is significantly less powerful on a molecule by molecule basis than the natural compound.

What did they find?
One of the striking results found by Ilonka et al. is that several of the flame retardants studied are more powerful than the natural hormone, T4, in binding with transthryetin:

 

"The results presented in this study clearly demonstrate for the first time that hydroxylated brominated flame retardants of several different classes are able to bind to human transthyretin in vitro, some with extremely high potency, e.g., TBBPA and PBP. This is an important finding, as brominated flame retardants are used extensively at present for a large variety of applications and can be detected in wildlife and humans. The results of this paper thus indicate the possible capability of a large group of particularly brominated industrial chemicals to interfere with and potentially disrupt the thyroid hormone transport and metabolism."