# Periodontal Condition and Subgingival Microbiota Characterization in Subjects with Down Syndrome
## 4 Discussion
Both techniques are restricted in terms of target bacterial species, in this case three for qPCR and ten for culture. Other techniques may have a wider target such a checkerboard DNA-DNA hybridization: in the study by [PERSON] et al. [23], 14 species were targeted and _T. forysthia_ was identified as a relevant pathogen in all age cohorts, while _P. gingivalis_, _C. rectus_, _P. intermedia_ and _A. actinomycetemcomitans_ were relevant (among others) in the older cohorts; conversely, in the study by [PERSON] et al. [19], 40 species were targeted and _Treponema socrankii_ showed significantly higher levels in DS subjects with periodontitis, as compared to individuals without periodontitis. These restrictions in the number of target species can be solved today using Next Generation Sequencing approaches, as shown in a publication based on subset of samples of the present study, but only comparing periodontitis and healthy periodontal subjects [36]: significant differences were observed at different taxonomical levels, with periodontitis patients demonstrating not only higher levels of periodontal pathogens, including _Tannerella_, _Treponema_, _Porphyromonas_ and _Aggregatibacter_ but also of new putative pathogens, such as species of _Peptostreptococcus_, _Filifactor_, _Freibacterium_ and _Desulfobulbus_; subjects with periodontal health showed more frequently species of _Veillonella_, _Neisseria_, _Gamella_ and _Granulicatella_. Different case definitions have been used to assess the periodontal status of DS subjects. For example, [PERSON] et al. [16] defined four categories as sound (CAL, 0 to 1 mm), mild periodontitis (CAL, 2 to 3 mm), moderate periodontitis (CAL, 4 to 5 mm) and severe periodontitis (CAL, 6 mm or more); while [PERSON] et al. [19] defined periodontitis as 5% or more teeth with CAL of 5 mm or more. In the present study, a more solid reference, the one proposed by the 2018 Classification of Periodontal and Peri-Implant Diseases and Conditions [17,28,29], was applied, with some modifications for the optimal use of the registered variables. The significant differences in PD, CAL and BOP among the studied categories were expected, since those variables were part of the case definition. For PII, closely associated with BOP, it should be highlighted the large magnitude of the differences between periodontal health (around 45%), when compared with gingivitis and periodontitis (over 70%). The influence of this finding in the overall results deserve a careful analysis and the influence of a poor plaque control in the onset of gingivitis and periodontitis should be considered. In light of the possible impact of the limited skills in supragingival biofilm control in DS individuals, this factor should be taken into consideration [11]. In the present study, patients with periodontitis presented a significantly lower prevalence of hypothyroidism (10%) and the use of substitute medication (levothyroxine), compared with periodontal health and gingivitis subjects (42-44%). Hypothyroidism is the most common pathological hormone deficiency and, among the known risk factors for developing hypothyroidism, DS has been listed, usually as an autoimmune thyroiditis [46], which is consistent with the higher prevalence of autoimmune diseases in DS. Thus, the estimated prevalence of thyroid disorders in DS subjects reaches 40% in some series [47] and thanks to systematic screening, usually hypothyroidism is detected in subclinical stages in DS individuals [48], being subclinical hypothyroidism the most common detected thyroid abnormality in DS subjects [49]. Since the rate of conversion to overt hypothyroidism has been reported to be low in follow up studies [50], treatment of subclinical hypothyroidism is only advised by most authors in case of conversion to overt hypothyroidism [51].