Impact of environmental pollutants on vertebrate thyroid systems

Thyroid hormones play an important regulatory role in the metabolism,growth,development and reproduction of vertebrates.Among the increasing environmental pollutants,many of them are endocrine disruptors.Thyroid gland is very sensitive to the environmental pollutants.The pollutants can bring on many changes thyroid gland,such as the follicular size and number,colloid content in follicular cavity,synthesis,secretion and transportation of thyroid hormones,expression of a variety of enzymes in the metabolism,and changes of enzyme activity.They also bind competitively to thyroid hormone receptors and disrupt various physiological functions regulated by thyroid hormones,and even lead to death.As the metamorphosis of amphibian is controlled by thyroid hormones directly,tadpole has been recognized as the first type of pollution indicating animals in bioassay of screening and study of environmental endocrine disruptors.