decreasing among adults, increasing among infants, unexplained geographic disparities

In France, over the period 2014-2019, the incidence of acquired hypothyroidism marked a clear decline. Congenital forms of this pathology, with potentially serious consequences in terms of development, are on the other hand increasing among female newborns. The reasons for this increase are still unclear, although the involvement of environmental pollutants cannot be ruled out.

Persistent fatigue, unexplained weight gain or difficulty concentrating are all symptoms of hypothyroidism. This disorder occurs when the thyroid gland does not produce enough thyroid hormones. In France, it affects around 2% of the French population.

In order to take stock of the situation, we carried out an inventory of the incidence, that is to say the number of new cases occurring over a given period, of hypothyroidism in France over the period 2014. -2019, by analyzing data from the National Health Data System.

Our results showed a decrease in hypothyroidism in adults over this period, but an increase in newborns, particularly females.

What is hypothyroidism?

The thyroid is a small butterfly-shaped gland 5 cm in diameter located in the superficial part of the neck, in front of the trachea at the level of the larynx. Modest in size, it nevertheless plays a major role in our body. It produces thyroid hormones, which are involved in the development, growth and metabolism of the majority of tissues in the human body.

As a result, a deficiency of thyroid hormones, or hypothyroidism, can have serious consequences on the health of the people concerned. Such a deficit can occur in adulthood, or be congenital, in other words be present from birth.

In adults, the causes of hypothyroidism can be diverse. Globally, iodine deficiency is the leading cause of hypothyroidism (usually in low-income countries). In countries with a higher socio-economic level such as France, where the diet normally contains sufficient iodine if iodized salt (and not artisanal salt) is consumed, the main cause is an autoimmune disease, thyroiditis. of Hashimoto (the immune system attacks the body).

Forms of congenital hypothyroidism can be either transient (meaning that the child only needs treatment early in life) or, more frequently, permanent. In two thirds of these cases, the thyroid gland has an abnormal appearance or is absent, and treatment must be taken for life.

A deficiency in thyroid hormones can result in delays in growth and development (particularly of the brain), hypothyroidism is systematically looked for in newborns on the 3rd day of life.

A decreasing number of cases among adults

Our work demonstrated a significant decrease in the incidence of hypothyroidism in adults of both sexes between 2014 and 2019.

In women, the incidence of hypothyroidism decreased from 535.7 per 100,000 people per year in 2014 to 335.5 in 2019. In men, its incidence decreased from 197.5 to 141.7 per 100 000 people per year over the same period. This phenomenon has also been observed very recently in the United Kingdom.

Between the ages of 15 and 75, we see that the incidence of this disease in women is two to three times higher than in men. The female predominance of autoimmune diseases such as Hashimoto’s thyroiditis is known and is due to several combined phenomena.

The frequency of hypothyroidism also increases with the age of individuals. It must be emphasized that the origin of hypothyroidism is multifactorial: it is not only autoimmune, but can also be the result of deficiencies, or more rarely of medical treatments (so-called “iatrogenic” hypothyroidism). Furthermore, the TSH level increases as we age, which reflects the aging of the thyroid gland.

Newborns: an increase in hypothyroidism in girls

In 2014, the incidence of transient hypothyroidism was 17.9 per 100,000 births in girls and 18.4 in boys. It changed little between 2014 and 2019.

The situation is different regarding the incidence of various forms of permanent congenital hypothyroidism in female newborns. Indeed, their frequency increased by 8.9% per year in France: from 36.9 cases per 100,000 births per year in 2014, it increased to 51 cases per 100,000 births per year in 2019.

Furthermore, the data analyzed indicate that permanent congenital hypothyroidism is 1.5 to 2 times less common in boys than in girls (38.3 per 100,000 births per year in boys in 2019). This increase phenomenon is also observed elsewhere in the world, although its origin remains a subject of debate among experts.

Marked geographical disparities in France

The distribution of cases of hypothyroidism, in adults and children, does not appear homogeneous across metropolitan France. We were able to highlight, in women, real clusters, that is to say distinct geographical groupings, where the incidence of hypothyroidism was higher.

In newborns, we were unable to search for clusters due to the rarity of the disease. We have nevertheless noted that, as in adults, certain (former) regions, such as Nord-Pas-de-Calais and Lorraine, are among the most affected by hypothyroidism.

How is this situation explained?

One of the main explanations for the reduced incidence of hypothyroidism in adults is the application of the recommendations for good diagnostic practice, issued in 2007. Their objective was to reduce the overtreatment of the crude hypothyroidism whose diagnosis is not always confirmed, in other words avoid the treatment being used excessively.

You should know that before this period, in approximately 30% of cases, treatment for hypothyroidism was initiated solely on the basis of clinical signs, without measuring thyroid-stimulating hormone, or TSH. Since 2007, doctors have been encouraged to make their diagnosis using a decision-making algorithm that uses the measurement of TSH (thyroid stimulating hormone).

Produced by the pituitary gland (a small gland located at the base of the brain), TSH regulates the secretion of thyroid hormones. When the thyroid doesn’t produce enough hormones, the pituitary gland increases production of TSH, which stimulates the thyroid. Conversely, when the thyroid produces too much hormone, the pituitary gland reduces TSH production, which slows down the production of thyroid hormones. An abnormally high TSH level can therefore be a sign of hypothyroidism.

Regarding congenital hypothyroidism, the causes are currently poorly understood. We know that genetics only explains about 5% of cases where the thyroid has an abnormal appearance or is absent.).

Environmental exposure to endocrine disruptors to blame?

In women, the increased frequency of permanent congenital hypothyroidism, as well as the geographic disparities observed, raise questions about the potential impact of environmental exposures on these phenomena.

Scientific studies reveal that there are associations between newborn TSH levels and prenatal exposure to various pollutants present in outdoor air or drinking water: nitrate, perchlorate, PM2.5 and PM10 particles, polychlorinated biphenyls (PCB), dioxins, organochlorine pesticides, etc.

At present, however, no study has examined the direct links between the incidence of hypothyroidism and environmental determinants in France. This is one of the objectives of our project [« Exposome et thyroïde » (EXPOTHYR)]winner of the joint call for projects from the Health Data Hub and Green Data for Health, 2nd wave.

In EXPOTHYR, the incidence data for hypothyroidism over 2016-2022, again extracted from the National Health Data System, will be cross-referenced with different parameters likely to influence the occurrence of the disease: water pollution, air quality, pesticide use, land use, revegetation, as well as various socio-economic indicators.

This project should lead to better knowledge of the determinants of the disease. Please note that all of the data and scripts used during this work, the results of which will also be the subject of scientific publications, will be made available to the community at the end of the project, in “open access”.