As is so often the case, evidence again appears emphasizing that oral health is just one part of overall health. In this case, low thyroid function (“hypothyroidism”) is seen as a potential case of problems in the mouth.
The First Link: Pesticides and Hypothyroidism
Recent studies have investigated the role of exposure to pesticides and herbicides in contributing to hypothyroidism.
A 20-year study of 35,150 North Carolina and Iowa farmers and their partners shows an association between pesticide exposure and increased risk of hypothyroidism. The study looked at eight insecticides and three herbicides and determined that high exposures to these chemicals was associated with a risk of hypothyroidism. However, while these results are generally consistent with prior studies, and sound very disturbing, the subjects of the study were exposed to very high levels of these insecticides and herbicides. Not only were the subjects farmers, they were “private pesticide applicators”, making them particularly highly exposed.
Second, while the study recognized a significant degree of correlation between pesticide exposure and the risk of hypothyroidism, it does not describe a mechanism to connect these two elements. Possible mechanisms can be considered, such as: 1) pesticides interfering with thyroid hormone synthesis, or 2) pesticides interfering with the transport and metabolism of thyroid hormones via immunomodulation or the development of autoimmunity. While these theories go beyond both the parameters of the study of farmers’ exposure to pesticides, as well as beyond the scope of this blog post, it is worth pointing out that further research might well establish rigorous connections.
So, while this study and others like it are not conclusive, a correlation does appear.
The Second Link: Hypothyroidism’s Effect on Overall Health
Both hypothyroidism (low thyroid gland function) and hyperthyroidism (overactive thyroid gland function) can cause many problems for oral health. In the case of hypothyroidism, such as that attributed to pesticide exposure in the study described above, signs and symptoms may include:
- Increased sensitivity to cold
- Dry skin
- Weight gain
- Puffy face
- Muscle weakness
- Elevated blood cholesterol level
- Muscle aches, tenderness and stiffness
- Pain, stiffness or swelling in your joints
- Heavier than normal or irregular menstrual periods
- Thinning hair
- Slowed heart rate
- Impaired memory
- Enlarged thyroid gland (goiter) 
As well as all these general health problems, hypothyroidism can have dental implications also.
The Third Link: Dental Implications of Hypothyroidism
It is a relatively short step from recognizing the effect of hypothyroidism on overall health to seeing the dental implications of hypothyroidism. Dental implications include dry mouth (“xerostoma”); weaker resistance to infection ultimately leading to inflammation of the gums; and eventual periodontitis. MV Health System notes, “Patients with hypothyroidism, also known as underactive thyroid, may experience macroglossia (enlargement of the tongue), gum disease, slow healing of mouth sores, and, in children, delayed tooth growth. Problems with hypoglycemia (low blood sugar) are common in patients with underactive thyroids and can cause an increased risk of mouth and tooth infections.
The Fourth Link: Reducing Pesticide Exposure
While the conclusions of this study on pesticides and hypothyroidism are not definitive, it is the case that consumers of organic produce have lower levels of these same pesticides than consumers of conventional food. As a general approach therefore, it makes sense to reduce our exposure to potential sources of harm. In the case of pesticides, that goal can be partially achieved by choosing organically grown food when possible.
It also makes sense to avoid or limit exposure to insecticides and herbicides through awareness of locations where pesticide concentrations can be particularly high, such as golf courses and highly managed gardens. That said, the risk of problems arising in these locations is small compared to the life of a professional pesticide applicator.
The risks hypothyroidism presents to most people at this point are mostly small and speculative. Certainly, eating organic produce may reduce your risk of hypothyroidism associated with pesticide exposure, and avoiding environments with high pesticide use is also wise. 
 Shrestha, Srishti et al. “Pesticide Use and Incident Hypothyroidism in Pesticide Applicators in the Agricultural Health Study.” Environmental health perspectives vol. 126,9 (2018): 97008. doi:10.1289/EHP3194. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6375417/.
 Mayo Clinic, “Hypothyroidism (underactive thyroid)”. Available from: https://www.mayoclinic.org/diseases-conditions/hypothyroidism/symptoms-causes/syc-20350284.
 Kothiwale S, Panjwani V. “Impact of thyroid hormone dysfunction on periodontal disease.” J Sci Soc [serial online] 2016 [cited 2019 Dec 17];43:34-7. Available from: http://www.jscisociety.com/text.asp?2016/43/1/34/175456.
 MV Health System. “Thyroid Problems and Oral Health.” Available from: https://mvhealthsystem.org/resource/thyroid-problems-and-oral-health.
 Ruscio, Dr. Michael, “Long-Term Study Links Pesticide Exposure to Hypothyroidism”. Available at: https://drruscio.com/study-links-pesticide-exposure-to-hypothyroidism/.