Bisphenol can deregulate thyroid hormones even at low doses

Study of compound present in plastics also evaluated effects of exposure to the glyphosate-based herbicide most widely used in Brazil.

By Karina Toledo  |  Agência FAPESP – The idea that one of the key concepts in toxicology, that the dose makes the poison, may not apply to endocrine disruptors has been reinforced by research performed by Brazilian scientists at the Federal University of São Paulo (UNIFESP).

Present in water, air, plastics, foods, cosmetics, drugs and many other places, these chemical compounds interfere with the functioning of human and animal hormones, impairing important functions such as energy metabolism, the immune response, neurological development and reproduction.

In experiments with rats, the UNIFESP group observed that two endocrine disruptors – bisphenol A and a glyphosate-based herbicide – could alter the regulation of thyroid hormones even at doses well below the limit considered safe by regulators, if exposure occurred during pregnancy and lactation or during puberty. 

The results of the research, which was supported by FAPESP, were presented in September by Maria Izabel Chiamolera at the 32nd Annual Meeting of the Federation of Brazilian Societies for Experimental Biology (FeSBE) in Campos do Jordão, Brazil.

“Recent studies suggest that in the case of endocrine disruptors, the lowest dose isn’t always the safest, since cell defense mechanisms may fail to detect it,” Chiamolera explained. “On the other hand, the exposure window appears to make a real difference. Embryonic development and lactation are the critical phases, as well as puberty, when major hormone changes are taking place in the organism.”

Regulatory decisions about the use of these substances should also consider the principles of endocrinology rather than just toxicology, in her view.

The experiments with rats that served as a basis for this finding were performed in partnership with researchers at the State University of the Center-West (UNICENTRO) interested in studying the effects of these substances on the reproductive organs of male animals.

In the first model, pregnant rats were given drinking water with small amounts of a glyphosate-based herbicide, the kind most widely used in Brazilian agriculture, between mid-pregnancy and the fifth day after giving birth. Male offspring were evaluated at 90 days of age, when they were considered to have reached mature adulthood.

The UNIFESP group measured blood levels of thyroid hormones and found no significant difference compared with the control group (which consisted of unexposed animals). However, expression of key genes for thyroid regulation was altered in both the hypothalamus and the pituitary, as well as in the liver and heart, which are highly susceptible to the action of thyroid hormones. 

The genes in question included DIO2, which encodes Type II iodothyronine deiodinase, an enzyme that mediates conversion of thyroid hormone T4 into its active form, T3, and the gene that encodes MCT8 (monocarboxylate transporter 8), which helps thyroid hormones enter cells.

“The gene deregulation pattern we observed recalls hypothyroidism, albeit with some different elements,” Chiamerola said. “What is most interesting is that when we evaluated blood metabolites – amino acids and lipids of various sizes – we also found a pattern similar to hypothyroidism, showing that this alteration in gene expression was reflected by the metabolic phenotype.”

The UNICENTRO researchers observed an increase in blood levels of testosterone and gonadotropins (hormones secreted from the pituitary that stimulate the gonads, or sex glands, to carry out their reproductive or endocrine functions). They also found that animals exposed to the herbicide entered puberty earlier than unexposed animals and in adulthood displayed hypersexual behavior (constant interest in copulating) compatible with elevated testosterone.

Exposure of male rats to the glyphosate-based herbicide during puberty, on the other hand, was found to reduce blood levels of testosterone, delay sexual maturation, and alter the structure of their seminiferous tubules (the exocrine portion of the testis, where the spermatozoa develop and through which they leave the gland).

Bisphenol A

The second study tested the effects on rats of exposure to bisphenol A, a compound present in the formulation of hard plastics such as polycarbonate and that can be transferred to food in the event of a sharp change in temperature (heating or freezing).

Male rats were exposed to one-tenth of the dose considered safe during puberty (from the 23rd to the 58th day after birth) and evaluated 108 days after birth. The UNIFESP group found high blood levels of thyroid-stimulating hormone (TSH), which is secreted by the pituitary, enters the bloodstream, and stimulates the thyroid to secrete the hormones T3 and T4. They also found decreased blood levels of T4 and increased levels of T3.

“This is a strange pattern,” Chiamolera said. “T3 and T4 levels are normally either both raised or both lowered. This hormone profile resembles that of people with a mutation in the gene that encodes the transporter protein MCT8. Such patients have severe neurological alterations.”

In the perinatal model, where exposure occurred in the same window as in the experiment with glyphosate and with even smaller doses of bisphenol A than in the peripubertal model, they observed reduced levels of TSH and increased levels of T3 and T4 in adult offspring, producing a hormone pattern similar to that of hyperthyroidism.

“Our analyses of gene expression and metabolites were compatible with the hormone alterations observed in blood, resembling hypothyroidism in the peripubertal model and hyperthyroidism in the perinatal model,” Chiamolera said.

The UNICENTRO group observed sperm alterations in rats exposed to bisphenol A during puberty, as well as alterations similar to those seen in patients with hypogonadism, a condition in which the gonads produce insufficient amounts of sex hormones. The symptoms of this dysfunction include diminished libido, impaired fertility, loss of muscle mass, and fat mass gain, among others.

The complete results of the experiments can be found in articles published in the journal Reproductive Toxicology in 2009 and 2011, and in the journal Toxicology in 2015 and 2017.

“Our next step is to understand the reasons for altered gene expression in adulthood in these animals that were exposed only indirectly and only in early life. Is the pattern of regulation altered for good? Will their offspring inherit the alteration? We plan to investigate these questions now,” Chiamolera said.