The Effect of Microplastics on Hormones

By: Emily Ha, Matthew Mowlazadeh, and Riti Radhakrishnan

Abstract

Microplastics impact hormones, including thyroid, growth, and sex hormones. The thyroid hormone is very important for the functioning of the human body, especially in terms of metabolism, growth, and development (Kim & Park, 2019). Research demonstrates that bisphenol plastics have a detrimental effect on thyroid hormone concentrations (Kim & Park, 2019; Lee et al., 2019; Xu et al., 2019). The research studies found that microplastics will negatively affect growth hormones in ways such as the growth of children, the metabolism of adults, and overall lifespan. As for sex hormones, the minute plastic particles have been found to cause disruption in hormonal imbalances, affecting reproductive functions and fertility (Yifan Hong et al., 2023). Long-term exposure to microplastics may pose risks to both human and animal populations (Yifan Hong et al., 2023).

Keywords: Microplastics, Hormones, BPA, bisphenols, thyroid, estrogen, fertility

Thyroid Hormone Disruption

Thyroid hormone controls the human body’s metabolism, playing a vital role in growth and development. Thyroid hormone is secreted from the thyroid gland and is composed of thyroxine (T4) and triiodothyronine (T3). In addition, thyroid-stimulating hormone (TSH) exists (Kim & Park, 2019).

BPA, also known as Bisphenol A, is a chemical that is used in the production of many plastics (Kim & Park, 2019). Bisphenols are an incredibly common class of chemicals used in plastic manufacturing (Kim & Park, 2019), and can have an impact on thyroid hormone levels.

Many studies have shown that there is an association between the presence of BPA and thyroid hormone levels (Kim & Park, 2019; Lee et al., 2019; Xu et al., 2019). One study used embryo-larval zebrafish and found that the presence of many bisphenols, including A, S, F, and Z, increased the time it took for the zebrafish eggs to hatch (Lee et al., 2019). This is relevant because thyroid hormone plays a large part in growth and development (Kim & Park, 2019). This study concluded that thyroid hormones were disrupted in the zebrafish eggs, causing an increased time for the zebrafish eggs to hatch (Lee et al., 2019).

Another study demonstrated that higher exposure to BPA caused thyroid disruptions which led to negative effects on neurological development in rats (Xu et al., 2019). In the study, one group of developing rats were exposed to BPA in their drinking water while the other group of developing rats was not exposed to BPA (Xu et al., 2019). The study then monitored levels of thyroid in the rats and measured cognitive performance of the rats through various tests (Xu et al., 2019). The study discovered that BPA exposure led to a change in concentration of thyroid hormone (Xu et al., 2019). As a result, the rat pups that were exposed to BPA had negative effects on their social behavior and spatial memory once they reached maturity (Xu et al., 2019).

Growth Hormone Disruption

Microplastics can be seen everywhere in the environment and have led to poor health

effects on humans. One important endocrine disruption caused by microplastics is their impact on growth hormone (GH). Growth Hormone is a key player in the control of metabolism, homeostasis, and growth. There are three mechanisms of disruption on growth hormone:: chemical leaching, physical interaction, and adsorption of endocrine disrupting chemicals. In chemical leaching, microplastics can release hazardous additives or other chemicals that imitate or disrupt growth hormone function. These chemicals have the ability to bind to hormone receptors or disrupt growth hormone function. Microplastics have the ability to physically interact with hormonal receptors in order to block GH binding. This can be seen as competitive inhibition where a substrate molecule is prevented in binding to the active site due to the competitive inhibitor blocking the site. As for the last mechanism, microplastics have the ability to adsorb substances that affect hormones found in the environment. The absorbed compounds will then be released by the microplastics upon digestion, which may cause endocrine disruption leading to interference with growth hormone.

Microplastics, including endocrine disruptors like bisphenol A (BPA) can negatively

impact both humans and plants. In the first study, researchers investigated the effects of bisphenol A (BPA) on the division of the root tip cells of gymnosperm Abies cephalonica Loudon. The results show how BPA disrupts microtubule arrays of all mitotic stages, which affects chromosomes and endoplasmic reticulum patterns. The study involved germinating seeds of the Greek endemic cephalonian fir in commercial peat for 20-25 days. The seedlings were then exposed to different concentrations of BPA for various durations. For the germination tests, seedlings were germinated on filter paper soaked with distilled water and 0.44 mM BPA. The study examined the various combinations of BPA concentrations on seedlings and found that the best concentration was 0.44 mM at 3,6, and 12 hours. Nuclear DNA was stained with propidium iodine to identify cell cycle stages. It was concluded that the plant mitotic apparatus microtubules can be very sensitive to BPA and the level of effect depends on which stage it is in in the cell cycle (Adamakis, 2016). They found that the effects were fatal for mitosis because it prevented the formation of the metaphase plate, segregation at the anaphase stage and caused abnormal cytokinesis events. This study goes to show how harmful BPA can be on plant root tip cells, specifically the gymnosperm Abies cephalonica Loudon.

In the second study, researchers examined how BPA affects soybean root growth by

analyzing changes in plant hormones. Phytohormones, which regulate plant growth and stress responses, play a key role in this process. However, how BPA exactly affects plant growth by regulating endogenous hormones remains unclear. They investigated the effects of BPA on soybean (Glycine max L.) and the root growth at three different stages. They then analyzed the mechanism which caused BPA to change the formation of the root growth by looking at its hormone levels (Li, 2017). The results show that low concentrations of BPA actually improved root growth (except at the seed-filling stage). However, BPA at moderate and high concentrations inhibited root growth. The results overall demonstrated that the effects of BPA on root growth were related to BPA-induced changes in hormone and performed differently at the three different growth stages.

The disruption of growth hormone by microplastics can cause very harmful effects on the

body. When microplastics alter hormone levels, growth and development in children, metabolism in adults, and lifespan can be affected. It is important to recognize that the effects of microplastics also impact plant life and their growth hormones. As mentioned, this issue is not limited to humans. It is important to address these issues because it will help with our understanding of microplastics on humans and plants.

Disruption of Sex Hormones: Estrogen

The impact of microplastics on estrogen levels and function is an area of growing concern in reproductive health research. Microplastics can mimic or block the actions of natural hormones, potentially leading to imbalances in estrogen levels. The ovaries are the primary source of estrogen in females. Exposure to microplastics, as indicated by the studies, may lead to damage to ovarian tissues and disruption of normal ovarian function. While limited research exists on this topic, studies conducted on rats and aquatic organisms have provided valuable insights into the adverse consequences of plastic particles on estrogen levels (Sana Ullah et al., 2023)

Research indicates that the accumulation of microplastics in the ovaries and granulosa cells of rats can disrupt estrogen hormone regulation (Sana Ullah et al., 2023). Microplastics have been found to hinder follicle growth, decrease the levels of anti-Mullerian hormone (AMH) and estradiol, and cause irregular estrous cycles and abnormal folliculogenesis (Sana Ullah et al., 2023). Granulosa cells, crucial for ovarian development and follicle maturation, play a pivotal role in these processes. Moreover, microplastics made of polystyrene (PS) have been shown to induce ovarian fibrosis through the activation of the Wnt/β-Catenin signaling pathway, leading to apoptosis of granulosa cells and oxidative stress. This ultimately reduces the normal ovarian reserve capacity in rats (Sana Ullah et al., 2023).

Microplastics, acting as transport media for endocrine-disrupting compounds (EDCs),

have been associated with various endocrine disorders. These include infertility, precocious puberty, hormone-based tumors, metabolic problems, disruption of granulosa cell steroidogenesis, and polycystic ovary syndrome (PCOS) (Sana Ullah et al., 2023). Plastic additives such as PBDEs, BPA, phthalates, nonylphenols, octyl phenols, and biocides like TBT, mercury, arsenic, copper, cadmium, and lead are known to transfer from pregnant women to the fetal bloodstream, potentially causing neurodevelopmental abnormalities in infants (Sana Ullah et al., 2023).

Studies have revealed that polystyrene microplastics (PS-MPs) can accumulate in the yolk sacs of female fish embryos and their eggs, leading to disruptions in the normal physiology of offspring. The presence of PS-MPs in the ovaries of Oryzias melastigma has been linked to a reduction in key antioxidant enzymes, including superoxide dismutase, catalase, glutathione S-transferase, and glutathione peroxidase, indicative of oxidative stress(Sana Ullah et al., 2023). Moreover, studies have demonstrated that MPs can disrupt the hypothalamic-pituitary-gonadal (HPG) axis by down-regulating the transcription of genes involved in steroidogenesis, such as GnRH, vitellogenin, and choriogenin. These findings emphasize the potential of microplastics to interfere with estrogen regulation in aquatic organisms, raising concerns about reproductive health and ecosystem stability (Sana Ullah et al., 2023).

Although our understanding of the impact of microplastics on estrogen hormone is still evolving, existing evidence suggests that microplastics can disrupt estrogen levels through various mechanisms, including direct effects on the ovaries and the release of endocrine-disrupting compounds (Yifan Hong et al., 2023). Further research is required to comprehensively examine the effects of microplastics on estrogen hormone regulation in humans and wildlife (Sana Ullah et al., 2023). Addressing microplastic pollution is crucial for safeguarding hormonal balance and reproductive health, prompting the need for effective mitigation strategies and increased awareness of this emerging issue.

Conclusion

In summary, microplastics have a negative effect on hormones. Microplastics disrupt thyroid hormone, leading to many negative effects such as adverse effects on social behavior and spatial memory (Kim & Park, 2019; Lee et al., 2019; Xu et al., 2019). In addition, microplastics affect growth hormones by causing disruptions in the mitosis cycle and inhibiting growth. Sex hormones are negatively affected because microplastics interfere with the production, release, and function of sex hormones such as estrogen, progesterone, and testosterone (Yifan Hong et al., 2023). This disruption can lead to hormonal imbalances, which affects reproductive processes, sexual development, and fertility in both humans and animals (Sana Ullah et al., 2023). The amount of microplastics must be limited so hormones in humans and other living organisms can regulate and work negatively without any disruptions.

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