The endocrine system is vulnerable to a range of chemicals in the environment. Endocrine disrupting chemicals (EDCs) are exogenous agents that can induce responses on the endocrine system because of their hormone-like activity and toxicity. Specific to this study are thyroid disrupting chemicals (TDCs), these are EDCs that specifically disrupt the thyroid hormone signalling pathway, and this may result in adverse health effects. Thyroid hormones play a crucial part in metabolism, growth, maintenance of brain function and fertility; hence disruption of the thyroid signalling axis implicates human health. We are exposed to TDCs regularly, and studies have shown an association between TDC exposure and neurobehavioural disorders, reproductive abnormalities and obesity. There is a lack of data associated to thyroid hormone receptor activity in surface and drinking water. Hence, the potential human health risks posed by thyroid disruption may therefore be underestimated. The aim of the study was to optimise and validate the GH3.TRE.Luc reporter gene bioassay that can measure thyroid hormone receptor mediated activity and cytotoxicity in drinking and source water, with relevance to water monitoring. The GH3.TRE.Luc reporter gene bioassay was established, optimized and validated to detect thyroid hormone receptor activity. The luciferace assay was used to test for metabolic activity and the resazurine cell proliferation assay was used to assess cell viability. The assay was applied to compounds with agonistic and antagonistic properties; triidothyronine (T3), thyroxine (T4), triac, tetrac, amiodarone, sodium arsenite, pentachlorophenol (PCP), ethylene thiourea, 2,2,4,4-tetrahydroxybenzophenone (THBP) and methimazole. It was also applied to environmental and drinking water samples from the Global Water Research Coalition (GWRC). Finally, the assay was applied to 48 water samples from a water treatment plant in South Africa, collected over a period of 12 months. Every month, four samples were collected. Two samples were source water samples, with one going into the treatment plant and coming out as 2 distribution pipelines (drinking water). For optimisation and validation, the dose response curves obtained for T3, T4, tetrac and triac (agonists) were comparable to literature. Antagonistic behaviour was seen in sodium arsenite, amiodarone, PCP and methimazole. Spiked water samples from the GWRC showed thyroid hormone receptor activity. Sixteen of the 48 water samples collected from the water treatment plant were positive for thyroid hormone disruptor activity. Highest activity was seen in the winter season, accounting for seasonal variations. High TDCs activity reported in the source water may be due to activities occurring near the dam. The water treatment plant seemed effective for only one of the distribution pipelines, and not the other. This study confirms that GH3.TRE.Luc Reporter Gene Bioassay is a sensitive and effective tool to identify and quantify TDC activity in pure chemicals and in complex environmental mixtures present in water. Further monitoring of water sources for TDCs is recommended to ensure water quality and safety. / Dissertation (MSc)--University of Pretoria, 2017. / School of Health Systems and Public Health (SHSPH) / MSc / Unrestricted