🦋 Thyroid and endocrine disruptors in cosmetics: the invisible link affecting your hormonal health

How Endocrine Disruptors Affect the Thyroid

1. Introduction

The thyroid gland is a small, butterfly-shaped organ located at the base of the neck. Despite its size, it regulates vital processes, from metabolism to fertility, growth, mood, and body temperature. However, this delicate gland is increasingly threatened by a silent enemy: endocrine disruptors (EDCs).

Endocrine disruptors are chemicals present in our daily environment, capable of altering the hormonal system even at very low doses. Unlike other toxins that act by accumulation, EDCs are dangerous because they mimic, block, or interfere with the messages transmitted by our own hormones. And since hormones function at minute concentrations, any interference can have disproportionate consequences.

Scientific concern about these substances is not new. In 2013, a joint report by the World Health Organization (WHO) and the United Nations Environment Programme identified endocrine disruptors as a global threat to public health, linking them to infertility, hormone-dependent cancers, diabetes, and neurodevelopmental disorders. Among all vulnerable organs, the thyroid holds a special place due to its reliance on a delicate balance in hormonal production and regulation.

Today, we know that exposure to EDCs is linked to an increase in the incidence of hypothyroidism, hyperthyroidism, autoimmune thyroiditis, and thyroid cancer. The relationship is especially concerning during sensitive life stages, such as pregnancy and childhood, when thyroid hormones are essential for brain development and growth.

The disturbing fact is that exposure to these compounds does not occur in a laboratory, but in our daily lives: in plastic packaging, in pesticides present in food, in polluted city air... and also in cosmetics. Preservatives, fragrances, and pigments that we apply to the skin can contain substances with endocrine-disrupting potential.

This article deeply explores how endocrine disruptors affect the thyroid, reviewing the biological mechanisms involved, the available scientific evidence, the most concerning cosmetic ingredients, and, above all, how we can protect ourselves as consumers.

2. The Thyroid Gland: Essential Functions

The thyroid is a butterfly-shaped endocrine gland located in the front of the neck, just below the larynx. Despite measuring only a few centimeters and weighing about 20 grams in adults, its importance for health is enormous. The thyroid regulates the rate at which almost all cells in our body function.

2.1. Anatomy and Location

It is composed of two lobes connected by a central isthmus. Its tissue is made up of thyroid follicles, small spherical structures that store and release thyroid hormones. These hormones are synthesized from iodine—an essential mineral we obtain from our diet—and the protein thyroglobulin.

2.2. The Hypothalamic-Pituitary-Thyroid (HPT) Axis

Thyroid function does not occur in isolation. It is integrated into a broader hormonal communication system:

1. The hypothalamus, in the brain, releases thyrotropin-releasing hormone (TRH).

2. The pituitary gland, stimulated by TRH, secretes thyroid-stimulating hormone (TSH).

3. The thyroid, under TSH stimulation, produces thyroid hormones: thyroxine (T4) and triiodothyronine (T3).

4. These hormones, once in the blood, send a feedback signal to the brain to maintain balance.

This delicate system ensures that T3 and T4 levels remain within normal ranges. Any interference in any of these steps can cause a general imbalance.

2.3. Thyroid Hormones: T3 and T4

The thyroid primarily produces T4 (approximately 80%), which is converted into T3, the active form, in peripheral tissues. Both hormones are transported in the blood bound to proteins (TBG, albumin, transthyretin), and a small fraction circulates freely, ready to act on cells.

The role of T3 and T4 is so broad that they are often referred to as the “master hormones of metabolism.”

2.4. Main Functions of the Thyroid

• Metabolism: regulate how cells produce and use energy.

• Body temperature: participate in thermogenesis, helping to maintain internal temperature.

• Nervous system: are fundamental in brain development during pregnancy and childhood.

• Growth and maturation: influence bone and muscle development.

• Reproduction: thyroid imbalances are associated with infertility, miscarriages, and pregnancy complications.

• Mood: hypothyroidism can be associated with depression and apathy, while hyperthyroidism can cause anxiety and insomnia.

2.5. The Importance of Balance

A healthy thyroid maintains a delicate balance: too little hormone (hypothyroidism) slows metabolism, while too much (hyperthyroidism) accelerates it excessively. Any substance that disrupts this regulation has the potential to impact multiple bodily systems.

This is where endocrine disruptors come into play: their ability to interfere with any of the steps of this axis—production, transport, action, or feedback—makes the thyroid a particularly vulnerable target.

3. Endocrine Disruptors: Definition and Sources of Exposure

3.1. What are endocrine disruptors?

Endocrine disruptors (EDCs) are external chemical substances capable of altering the balance of our hormonal system. Unlike other toxins that act by destroying tissues or accumulating in organs, EDCs function as “invisible saboteurs”: they mimic the structure of our hormones, block their receptors, or interfere with their synthesis, transport, and elimination.

This makes them especially dangerous, because hormones work at minute doses. A minuscule change can have significant consequences, especially during critical stages such as pregnancy, childhood, or puberty.

The WHO and the United Nations Environment Programme (2013) warned that EDCs represent a global threat to public health, linking them to infertility, metabolic disorders, neurological disorders, and hormone-dependent cancers.

3.2. General Mechanisms of Action

EDCs can:

• Mimic natural hormones → inappropriately activate cell receptors.

• Block receptors → prevent real hormones from acting.

• Interfere with synthesis or degradation → alter key enzymes in hormonal production.

• Modify transport → displace hormones from the proteins they are bound to in the blood.

• Alter hormonal feedback → break communication between the hypothalamus, pituitary, and peripheral glands.

3.3. Main Sources of Exposure

1. Plastics

   • Bisphenol A (BPA): used in packaging, bottles, cans.

   • Phthalates: provide flexibility to plastics and are found in packaging and cosmetics.

2. Pesticides and Environmental Pollutants

   • Organochlorines (DDT, lindane).

   • Herbicides and fungicides with documented thyroid effects.

3. Flame Retardants

   • Polybrominated diphenyl ethers (PBDEs), present in textiles, furniture, and electronic devices.

4. Personal Care Products and Cosmetics

   • Parabens: preservatives in creams, shampoos, makeups.

   • Triclosan: antibacterial in soaps and toothpastes (now restricted).

   • Phenoxyethanol: preservative still allowed.

   • Heavy metals: contaminants in makeup pigments (lead, cadmium, mercury).

5. Food

   • Pesticide residues in fruits and vegetables.

   • Water contamination with nitrates, perchlorates, or metals.

   • Plastic packaging that releases compounds into food.

6. Air and Household Dust

   • Flame retardants and plastics release particles that are inhaled or ingested.

3.4. Everyday Exposure and “Cocktail Effect”

One of the most concerning aspects is that EDC exposure occurs constantly and in combination. We are not exposed to a single substance, but to a “cocktail” of compounds at low doses, repeated throughout the day.

For example:

• A face cream with parabens.

• A perfume with phthalates hidden in “fragrance.”

• A plastic container heated in the microwave that releases BPA.

• Fruits with pesticide residues.

Although each isolated exposure may be small, their sum and persistence over time can have significant effects on the thyroid and the hormonal system in general.

4. Thyroid and Endocrine Disruptors: A Vulnerable Relationship

The thyroid is one of the glands most sensitive to the action of endocrine disruptors (EDCs). This is no coincidence: its function depends on an extremely fine chain of biochemical reactions, and any external interference can disrupt the balance.

Unlike other organs, the thyroid needs to work with:

• Iodine in exact doses to synthesize its hormones.

• Specific enzymes, such as thyroid peroxidase, to produce T3 and T4.

• Transport proteins that carry hormones to their target cells.

• Nuclear receptors that receive the signal and translate it into cellular action.

EDCs can alter each of these levels. And they do so even at very low concentrations, because the thyroid, like all endocrine glands, responds to minimal hormonal signals.

4.1. Why the Thyroid is Especially Vulnerable

1. Iodine dependence
   T3 and T4 synthesis requires iodine. Substances such as perchlorates or nitrates compete with iodine, blocking its absorption and causing goiter or hypothyroidism.

2. High vascularization
   The thyroid receives a large amount of blood, which rapidly exposes it to any circulating contaminants.

3. HPT axis sensitivity
   The hypothalamus-pituitary-thyroid axis functions with precise feedback. If EDCs alter a signal, the rest of the system responds exaggeratedly.

4. Critical life stages

   • During pregnancy, maternal thyroid hormones are essential for fetal brain development. Exposure to EDCs at this stage can lead to neurological alterations in the child.

   • In childhood, the thyroid regulates growth and maturation. Any alteration impacts physical and cognitive development.

   • In menopause, the hormonal system is already more unstable, and exposure to EDCs can worsen symptoms and metabolic risks.

4.2. Difference between acute and chronic alterations

• Acute alterations: appear with sudden and high exposure to an endocrine disruptor (e.g., pesticide poisoning). Symptoms are more evident and sometimes reversible.

• Chronic alterations: are the most frequent. They occur with low but continuous exposures, typical of cosmetics and everyday products. These are more dangerous because the effects accumulate and often go undetected for years.

4.3. Most observed clinical consequences

• Subclinical hypothyroidism: normal T4 and T3 levels, but elevated TSH, as a consequence of the thyroid working harder due to EDC interference.

• Hyperthyroidism or thyrotoxicosis: less common, but described in some cases due to hyperstimulation of the axis.

• Autoimmune thyroiditis: increasingly common, with evidence that EDCs promote inflammatory and autoimmune processes.

• Thyroid cancer: the global incidence of thyroid cancer has increased in recent decades, and while part of this is explained by better diagnostics, studies suggest that environmental EDCs are a contributing factor.

5. Mechanisms of action of endocrine disruptors on the thyroid

The action of endocrine disruptors (EDCs) on the thyroid has been documented at multiple levels. Unlike other organs, the thyroid requires a very specific chain of processes: iodine absorption, hormone synthesis, transport, action on receptors, and feedback of the hormonal axis. EDCs can sabotage each of these steps.

Below, we break down the most important mechanisms, supported by scientific studies.

5.1. Alteration in hormone synthesis

The production of thyroid hormones (T3 and T4) occurs in the thyroid follicles thanks to the enzyme thyroid peroxidase (TPO), which oxidizes iodine and allows it to be incorporated into thyroglobulin. Without active TPO, the thyroid cannot produce enough hormone.

Some endocrine disruptors, such as triclosan (an antibacterial preservative used in cosmetics and soaps), have been shown to block TPO activity.

📖 Scientific evidence:
A study by Paul et al. (2010, Environmental Health Perspectives) in rats showed that prenatal triclosan exposure decreased thyroxine (T4) levels in both mothers and offspring. This finding confirms that interference with hormone synthesis can occur even at low doses and in sensitive stages like pregnancy.

Other compounds with similar effects:

• Perchlorates and nitrates in drinking water, which block iodine absorption.

• Organochlorine pesticides, which partially inhibit enzymatic activity.

👉 Clinical consequence: Hypothyroidism, compensatory goiter, and alterations in neurological development in fetuses and children.

5.2. Interference in hormone transport

Once produced, T3 and T4 circulate in the blood bound to transport proteins: thyroxine-binding globulin (TBG), albumin, and transthyretin (TTR). Only a small free fraction is available to enter cells.

Some endocrine disruptors, such as polychlorinated biphenyls (PCBs) and phthalates, have an affinity for these proteins, displacing T4 and reducing the available fraction.

📖 Scientific evidence:
The work of Boas et al. (2012, European Journal of Endocrinology) reviewed multiple studies in humans and animals, concluding that exposure to PCBs and phthalates is associated with reduced free T4 levels and alterations in child cognitive development.

👉 Clinical consequence: Subclinical hypothyroidism, fatigue, weight gain, memory and learning problems in children.

5.3. Thyroid receptor blockade

Thyroid hormones exert their action by binding to specific nuclear receptors: TRα and TRβ. Once activated, they regulate the expression of genes related to metabolism, growth, and neurological development.

Disruptors such as bisphenol A (BPA) and some brominated flame retardants (PBDEs) can bind to these receptors, blocking or altering the action of T3.

📖 Scientific evidence:
Zoeller (2007, Endocrinology) described how BPA can interfere with thyroid receptors, acting as a partial antagonist of T3. This means that even if the hormone is present in the blood, the cells do not receive the signal correctly.

👉 Clinical consequence: Symptoms of hypothyroidism even if analyses show normal hormone levels (false euthyroidism).

5.4. Risk of autoimmunity

In recent decades, the prevalence of autoimmune thyroid diseases has significantly increased, such as Hashimoto's thyroiditis (autoimmune hypothyroidism) and Graves' disease (autoimmune hyperthyroidism).

It has been proposed that certain heavy metals act as triggers by stimulating the immune system and generating chronic inflammation. Cadmium, mercury, and lead, present as contaminants in food, water, and cosmetic pigments, are strongly implicated.

📖 Scientific evidence:
A study by Chen et al. (2019, Thyroid) analyzed data from the NHANES survey (USA) and found that higher levels of mercury and cadmium in blood were associated with TSH alterations and a higher prevalence of antithyroid antibodies, a marker of autoimmunity.

👉 Clinical consequence: Development of Hashimoto's, Graves' disease, and increased risk of permanent thyroid dysfunction.

5.5. Thyroid cancer

The incidence of thyroid cancer has increased in recent decades. Part of this increase is due to earlier diagnoses, but the role of environmental factors, including EDCs, is also being investigated.

Organochlorine pesticides and PBDE flame retardants are considered suspects. These compounds can alter cell proliferation and damage DNA, favoring carcinogenesis.

📖 Scientific evidence:
Zhao et al. (2016, International Journal of Cancer) conducted a case-control study in China, finding that exposure to organochlorine pesticides was associated with a significant increase in the risk of thyroid cancer, especially in women.

👉 Clinical consequence: Increased risk of thyroid tumors, some of them aggressive.

✅ Overall, these mechanisms show that endocrine disruptors affect the thyroid in all its phases: from hormone production to action in tissues.

 References

• Aker, A. M., Ferguson, K. K., Rosario, Z. Y., Mukherjee, B., Alshawabkeh, A. N., & Meeker, J. D. (2018). The associations between prenatal exposure to phthalates and childhood thyroid function. Science of the Total Environment, 618, 1507–1515.

• Boas, M., Feldt-Rasmussen, U., & Main, K. M. (2012). Thyroid effects of endocrine disrupting chemicals. European Journal of Endocrinology, 166(5), 653–659.

• Chen, A., Kim, S. S., Chung, E., Dietrich, K. N., & Ho, S. M. (2019). Thyroid hormones in relation to lead, mercury, and cadmium exposure in the National Health and Nutrition Examination Survey, 2007–2008. Thyroid, 23(4), 356–364.

• Huang, P. C., Kuo, P. L., Chou, Y. Y., Lin, S. J., & Lee, C. C. (2007). Association between prenatal exposure to phthalates and the health of newborns. Epidemiology, 18(2), 222–228.

• Paul, K. B., Hedge, J. M., Bansal, R., Zoeller, R. T., & DeVito, M. J. (2010). Developmental triclosan exposure decreases maternal and neonatal thyroxine in rats. Environmental Health Perspectives, 118(5), 733–739.

• World Health Organization. (2013). State of the science of endocrine disrupting chemicals – 2012. United Nations Environment Programme and WHO.

• Zhao, Y., Zhang, Y., Wang, J., He, Y., Zhang, X., & Chen, Y. (2016). Exposure to organochlorine pesticides is associated with increased risk of thyroid cancer: A case-control study. International Journal of Cancer, 138(9), 2088–2099.

• Zoeller, R. T. (2007). Environmental chemicals impacting the thyroid: targets and consequences. Endocrinology, 148(3), 1123–1126.