dentistry journal Article Potential Co-Factors of an Intraoral Contact Allergy—A Cross-Sectional Study Constanze Olms 1,2, * , Jana Schor 3 and Maryam Yahiaoui-Doktor 4 1 Private Practice, 29410 Salzwedel, Germany 2 Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany 3 Department of Molecular Systems Biology, Helmholtz Centre for Environmental Research GmbH—UFZ, 04103 Leipzig, Germany; jana.schor@ufz.de 4 Institute for Medical Informatics, Statistics and Epidemiology (IMISE), University of Leipzig, 04103 Leipzig, Germany; maryam.yahiaoui@imise.uni-leipzig.de * Correspondence: constanze.olms@medizin.uni-leipzig.de or constanze-olms@gmx.de; Tel.: +49-341-3550350; Fax: +49-341-3550399 Received: 14 June 2020; Accepted: 22 July 2020; Published: 3 August 2020   Abstract: The aim of this cross-sectional study was to evaluate the frequency of dental allergens and potential co-factors, especially hypothyroidism, for patients with an intraoral contact allergy. From 2015 to 2016, patients with confirmed symptoms of an intraoral contact allergy (study group SG n = 50) were recruited in the dental clinic of the University of Leipzig. The participants of the control group (CG n = 103) were patients without oral diseases or intraoral symptoms of a contact allergy. For the data collection, a new “Allergy questionnaire” was developed. Information on allergies and general diseases were collected. The statistical analysis was carried out with SPSS 23.0. Sensitizations/allergies to metals and composites were higher in SG compared to CG. Of all study participants (n = 148), 14.2% (n = 21) had a nickel allergy. In 18% (n = 8) of the SG a cobalt allergy based on all metal allergens could be seen. In addition, an association between a nickel and cobalt allergy was found. Hypothyroidism occurred significantly more frequently (p = 0.049) in SG than in CG. Sensitizations and allergies can occur to metals in dental alloys. Hypothyroidism increased the risk of having an allergy threefold. Keywords: contact allergy; metal sensitization; hypothyroidism 1. Introduction Dental materials are among the most common artificial materials that are incorporated into the human body. Intolerances to dental materials can either be directly associated with toxic damage or occur as part of sensitization or allergy. In recent decades, an increase in allergies could be observed in the world population [1–7]. This increase is attributed to lifestyle and cultural, economic, ecological and other factors [1]. In the German population, the number of allergies has been growing continuously for years [8]. Almost a third of the population in Germany is affected by an allergic disease in the course of their life and allergic sensitization has been found in 50% of the German population [8]. In the 2013 study on Adult Health in Germany (DEGS1), nearly 20% of the respondents said that they had an allergic disease in the last 12 months. The most common allergic diseases were hay fever (14.8%), bronchial asthma (8.6%) and contact dermatitis (8.1%) [9]. Contact eczema is also increasingly observed in children [10–12], and is also increasingly prevalent among the elderly [13,14]. The most important risk factors for a contact allergy are: workload, age, gender, the handling of consumer goods and a genetic predisposition [15]. Allergic contact dermatitis has thus reached a frequency similar to that of diabetes mellitus [16]. Dent. J. 2020, 8, 83; doi:10.3390/dj8030083 www.mdpi.com/journal/dentistry