Could Fluoride Be Harming Your Thyroid? What the Science Really Shows

When we talk about Hashimoto's and hypothyroidism, most people immediately think of thyroid hormones, genetics or autoimmunity.

But modern science has shown that a number of environmental factors can also influence thyroid health. Among them is a substance that is part of daily life for millions of people: fluoride.

Fluoride is widely used to prevent cavities and is found in treated tap water, toothpaste, mouthwash and many processed foods.

But can it interfere with thyroid function? The answer is more complex than a simple yes or no.

The relationship between fluoride and iodine

To follow this discussion, we need to understand why iodine matters. Iodine is an essential mineral for producing the hormones T4 (thyroxine) and T3 (triiodothyronine). Without enough iodine, the thyroid can't make its hormones efficiently.

Fluoride belongs to the same chemical group as iodine on the periodic table: the halogens. For decades, researchers have been investigating whether high fluoride exposure could interfere with the mechanisms the thyroid gland uses to take up and use iodine.

Experimental studies suggest that fluoride may affect the sodium-iodide symporter (NIS), the protein responsible for bringing iodine into thyroid cells. The literature also describes possible effects on:

• Iodine uptake
• Hormone synthesis
• Hormone conversion
• Cellular oxidative stress
• Genetic regulation of thyroid function

This doesn't mean that any contact with fluoride causes hypothyroidism. It means that, under certain conditions, the total exposure load can become clinically relevant.

What do the key studies show?

Study 1 — the largest meta-analysis published to date

In 2024, Iamandii and colleagues published a systematic review with a dose-response meta-analysis of studies on populations exposed to different levels of fluoride, pooling 24 studies with thyroid function data.

The authors found an association between higher fluoride exposure and higher TSH levels, with the effects most evident when the fluoride concentration in water exceeded roughly 2 to 2.5 mg/L. Children appeared to be more sensitive to the thyroid effects.

The review concluded that high fluoride exposure may be associated with changes in thyroid function, although the size of the effect varies across the populations studied.

Study 2 — a systematic review published in PLOS ONE

In 2024, Ferreira and colleagues carried out a systematic review focused specifically on the relationship between fluoride exposure and thyroid changes, searching multiple international scientific databases: of 3,568 articles initially identified, 7 studies met all the methodological criteria.

Five of the seven studies analyzed found an association between high fluoride exposure and changes in thyroid function. The authors concluded that there is a possible association between chronic high fluoride exposure and thyroid changes, but rated the overall quality of the evidence as low, stressing the need for more robust prospective studies.

Study 3 — the important Canadian study

One of the studies most often cited by supporters of water fluoridation was published in Canada in 2017 by Barberio and colleagues, with more than 6,000 participants, assessment of TSH and free T4, and measurement of fluoride in urine and in drinking water.

The authors found no significant association between fluoride exposure and changes in thyroid function in the population studied. But this result deserves a careful look: the Canadian population has relatively low exposure levels compared with regions of China and India, where many of the positive studies were conducted; the study used spot urine samples, which mainly reflect recent exposure rather than the load accumulated over decades; and it did not assess markers of thyroid autoimmunity, such as anti-TPO and anti-thyroglobulin.

So it doesn't directly answer the question that matters most to patients with Hashimoto's: might people with thyroid autoimmunity be more sensitive to fluoride's effects? The answer remains unknown.

The role of iodine deficiency

More recent studies suggest that the interaction between fluoride and the thyroid may depend on a person's nutritional status. People with lower iodine availability appear to be more susceptible to fluoride's possible thyroid effects. That makes biological sense: if a patient has iodine deficiency, selenium deficiency, Hashimoto's thyroiditis, chronic inflammation and increased oxidative stress, any factor that further reduces how efficiently iodine is used could potentially have a greater clinical impact.

Where do we encounter fluoride in everyday life?

Many people believe the main source is toothpaste. In reality, exposure tends to be cumulative. The main sources include:

• Fluoridated water — the main source for a large part of the population
• Toothpaste — especially when it is frequently swallowed
• Mouthwash — can add to the total exposure load
• Black tea and green tea — among the foods naturally richest in fluoride
• Sodas and other bottled drinks — depending on the water used to make them
• Processed foods — especially those made with fluoridated water
• Fish eaten with the bones — such as sardines and anchovies

So should we avoid fluoride completely?

Not necessarily. Fluoride has well-documented benefits in preventing tooth decay. On the other hand, the current scientific literature suggests that high, prolonged exposure may influence thyroid physiology in certain populations.

The central issue may not be toothpaste on its own. What science has been investigating is the total fluoride exposure load over a lifetime and how it may interact with iodine deficiency, genetic predisposition, chronic inflammation and autoimmune diseases.

What does this mean for people with Hashimoto's?

Patients with Hashimoto's often focus only on hormone replacement. However, clinical experience and the scientific literature show that a number of environmental factors can influence how the disease behaves, including: diet, gut health, vitamin D, selenium, iron, toxic metals, sleep, stress and exposure to environmental contaminants.

Fluoride may be one more piece of that puzzle. Not necessarily the cause of the disease, but a possible modulating factor in susceptible individuals.

How to assess this in practice

In the Functional Integrative Health approach, the goal is not to create fear around a single substance. The goal is to evaluate the patient broadly. In a more thorough workup, we may consider: history of environmental exposure, the quality of the water you drink, persistent symptoms despite a "normal" TSH, the presence of Hashimoto's and elevated antibodies, nutritional status for iodine, selenium, zinc, iron and vitamin D, and markers of inflammation, gut health and oxidative stress.

Each case needs to be interpreted individually. What is irrelevant for one person may be important for another, depending on their biological terrain.

Current science calls for balance, not extremes

Current science does not allow us to say that toothpaste causes Hashimoto's or hypothyroidism. On the other hand, it doesn't allow us to say that fluoride is completely irrelevant to thyroid health either.

The best available evidence suggests that high fluoride exposure may influence thyroid function parameters, especially in susceptible individuals and in iodine-deficient populations. For patients with Hashimoto's, the most important message is to understand that thyroid health depends on the interplay of genetics, immunity, nutrition and environment. And it is precisely in that interplay that Functional Integrative Health seeks to make a difference.

At the Dr. André Azevedo Clinic in Campinas, São Paulo, we evaluate Hashimoto's and hypothyroidism broadly, considering lab tests, symptoms, medical history, nutrition, gut health, inflammation and environmental exposures.