It is well-known that the metamorphosis process in amphibians is dependent on thyroid hormones. Laboratory studies have shown that several environmental contaminants can affect the function of thyroid hormones leading to alterations in the amphibian metamorphosis. The basic idea of the present study was to elucidate if the amphibian metamorphosis might be a useful tool as biomarker for effect-based environmental monitoring, examining wild tadpoles for potential thyroid hormone disruption. A laboratory test was performed to identify the responses from exposure to 6-propylthiouracil (PTU), which has a well-known mechanism on the thyroid system, on Swedish tadpoles from the Rana genus. This was followed by an environmental monitoring study where tadpoles of Rana arvalis, R. temporaria, and Bufo bufo were sampled from various sites in Sweden. Morphological data such as body weight, histopathological measurements of the thyroid glands, and environmental parameters were recorded. The results revealed that Rana tadpoles respond similar as other amphibians to PTU exposure, with interrupted development and increased size relative to the developmental stage. Data on some wild tadpoles showed similar features as the PTU exposed, such as high body weight, thus suggesting potential thyroid disrupting effects. However, histological evaluation of thyroid glands and pesticide analyses of the water revealed no clear evidence of chemical interactions. To a minor degree, the changes in body weight may be explained by natural circumstances such as pH, forest cover, and temperature. The present study cannot fully explain whether the high body weights recorded in some tadpoles have natural or chemical explanations. However, the study reveals that it is clearly achievable to catch tadpoles in suitable stages for the use in this type of biomonitoring and that the use of these biomarkers for assessment of thyroid disruption seems to be highly relevant.