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Estimation of risks associated with the occurrence of synthetic thyroid
PECH, Michal
The aim of this work was to provide a comprehensive overview of published findings regarding thyroid, thyroid hormones, their concentrations, stability in the aquatic environment and effect on fish. To determine the consumption of synthetic thyroid hormones in the Czech Republic, the data on the distribution of the drugs, in which they are contained, is analyzed over a period of five years (2011 - 2015). This has provided information about the issued amount of active ingredients i.e. levothyroxine (T4) and liothyronine (T3) for each year. T4 consumption increased over five years, except for 2013, with the highest consumption in 2015 corresponding to 34.6 kg of T4. T3 consumption ranged only in tens of grams before prescribing drugs containing T3 in the Czech Republic was ceased in September 2013. Based on the distribution of synthetic thyroid hormones, an estimate of concentrations that could normally occur in surface water was performed (PECRIVER). Two variants of the calculation were performed. The first variant was based on consumption of synthetic thyroid hormones. The second variant was based on excretion of thyroid hormones (including natural ones) by humans. Each of these variants was counted with the F value, i.e. the rate of removal of the chemical during the WWTP wastewater treatment process, from two different sources. The first source was the study by Svanfelt et al. (2010) where the efficacy of thyroid hormone removal during the WWTP purification process was 66% for T4 (this value was not determined for T3). The second source was EPI Suite software, which calculated the theoretical efficacy of removal of T4 and T3 at WWTP (35.83% for T4 and 5.36% for T3). The resulting concentrations were relatively low in the first variant. The median of the estimated concentrations of thyroid hormones in surface water with the efficacy of removing thyroid hormones during the WWTP purification process, taken from the study by Svanfelt et al. (2010), ranged from 0.6 - 0.7 ng-l-1 for T4. Using the theoretical value of the elimination rate calculated using the EPI Suite program, the median ranged from 1.1 to 1.3 ng-l-1 for T4 and from 0.0007 to 0.0025 ng-l-1 for T3 before it ceased to be used in September 2013. In the second variant, the resulting concentrations were somewhat higher. Median of the environmental concentrations of thyroid hormone with the removal rate taken from the study by Svanfelt et al. (2010) was determined to 1.5 ng-l-1 for T4. With the EPI Suite removal efficacy, the median was determined to 2.9 ng-l-1 for T4 and 0.13 ng-l-1 for T3. Estimated concentrations of thyroid hormones in surface water may currently seem relatively low to have a significant effect on fish, since the lowest thyroid hormone concentration at which any effect was observed ranged in tens of ng-l-1. However, it should be borne in mind that thyroid hormones enter the surface water as a result of incomplete removal during waste water treatment and fish are exposed to these hormones over a long period of time, meaning that even seemingly low concentration may pose a potential risk to aquatic organisms including fish. The fate of these hormones in the environment is therefore to be judged by further studies and toxicological experiments.
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