The Incidence of Autoimmune Thyroid Disease: A Systematic Review of the Literature

Anita McGrogan; Helen E. Seaman; John W. Wright; Corinne S. de Vries

Disclosures

Clin Endocrinol. 2008;69(5):687-696. 

In This Article

Method

Searches of the Medline (1980–2008), EMBASE (1980–2008) and ScienceDirect (1980–2008) databases were conducted using the keywords 'hypothyroidism', 'hyperthyroidism', 'thyrotoxicosis', 'Graves disease', 'thyroiditis' and 'incidence' or 'epidemiology'.

The inclusion criteria were that the studies reported original work, that a reasonable effort had been made by the authors to include all incident cases, and that the estimates of population size and person-time contributed were accurate. When assessing the likelihood of missing incident cases, papers were evaluated as follows. (1) For case-finding studies, did the authors ensure that all of the subjects contributing to incidence denominator data would have been eligible to have the disease diagnosed and did the authors check all relevant medical records? (2) For all studies, were cases checked to ensure that they were incident and not prevalent? (3) For all studies, did the authors ensure that the cause of thyroid disease was autoimmune and not secondary to another disease or environmental factors? Where possible, we only included incidence rates for cases of thyroid disease caused by autoimmunity, determination of which relied on information given in the paper.

The titles and abstracts of all of the studies produced by the searches were reviewed by two investigators and those papers accepted for inclusion in the study were appraised; inclusion of papers was discussed by three of the authors. Studies published in English, French, German, Spanish or Dutch were included. Review papers identified were searched for secondary references reporting on original research; secondary references found from any of the other papers reviewed were also included.

A standard data abstraction form was used to record all details of the papers reviewed; a copy of this is available on request from the authors. Each study was classified as being at low, medium or high risk for under- or overestimation of reported incidence rates by considering the reliability of numerator and denominator data. For instance, inclusion of prevalent cases or those thought not to be caused by autoimmunity will have led to overestimated rates, as will underestimated denominator data. Conversely, missing cases or an overestimated denominator (e.g. a catchment area from which not all inhabitants had access to hospital services) would be considered to result in underestimated incidence rates. Explanations provided by the papers' authors as to why incidence rates were as expected or whether they were considered to be an over- or underestimate of the true incidence rate were taken into account in this process. If the extent of likely error was considered to be very great, the study was excluded. To minimize subjectivity, this assessment was agreed between the authors and random checks were performed to ensure consistency. Rates are presented as the number of cases/100 000/year and, where sufficient data were given in the paper, rates were checked for accuracy.

Comments

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