An Estimation of the Incidence of Thyroiditis Among Girls in Primary Care in Spain.

Objective As other auto-immune processes, thyroiditis is monitored after vaccinations. We aimed to estimate the baseline incidence of thyroiditis among girls, before studying papillomavirus vaccination as a potential risk factor. Methods Observational cohort study including girls aged 9-18 years registered between 2002-2016 in the Spanish Primary Care Database for Pharmacoepidemiological Research (BIFAP). Girls were followed until a record of thyroiditis, 19 years of age, left the cohort, died, or the study ended. Anonymized records were reviewed for diagnosis confirmation (endocrine discharge letter and/or free-text comments) in a random sample. Incidence rate (IR/ per 105 py) was estimated. Results 480,169 girls made the cohort, of whom 641 had a record of thyroiditis (346 autoimmune thyroiditis; 17 thyroiditis of other types and 278 unspecified thyroiditis). Incidence of recorded thyroiditis increased with age from 23.96 in girls aged 9y to 47.91 in girls aged 14y, and stabilized around 31.06-34.43 among girls aged 15-18y. Of the 98 records reviewed, 60.2% were 'confirmed' thyroiditis cases, 32.7% 'possible' cases and 7.1% were discarded. After correction by discarded cases, IR was 20.83 'confirmed' thyroiditis cases/105 py, increasing up to 32.12 cases/105 py when 'confirmed' plus 'possible' cases were included. In years 2002-2005, incidences were lower (16.28 and 20.93 cases/105 py) than in the period 2007-2016 (21.17 and 33.78 cases/105 py) for 'confirmed' and 'confirmed' plus 'possible' respectively. Conclusions Two-thirds of the recorded thyroiditis included confirmatory evidence. The incidence of thyroiditis among girls increased with calendar years and age, and remained stable among aged 15-18 years.