Strengths and Limitations
The main strength of the study was the sizable long-term
population-based patient group included in our study. The advantages of
using the NHIRD such as long-term comprehensive follow-up and universal
coverage has been described in a previous study.(29) We excluded
patients with previous thyroid diseases due to possible interplay of a
history of thyroid diseases that might have caused Graves’ diseases. We
also qualified the definition of PCOS events in our study to more than
twice outpatient or one hospitalized record. Generally, our database
size ensures similar distributions due to well-balanced matching, and
reduces the heterogeneity and selection bias.
There are some limitations to our study. Since PCOS and autoimmune
thyroid diseases, including Graves’ diseases, have a familial clustering
phenomenon, an alternative explanation may be suggested to a common
genetic pathway.(30) However, the genetic background of individuals was
not available in our NHIRD scheme. The development of Graves’ diseases
may depend on different ethnic groups. Lab data such as hormone level
was also not available in NHIRD, posing limitations in further
addressing the association of the role of hormones in the association of
PCOS and Graves’ diseases. Although the Bureau of NHI uses strict
auditing mechanisms to reimburse insurance claims, the ICD-9 codes
claimed from the NHIRD might be inaccurate due to diagnostic uncertainty
and misclassification in reality. The symptoms of PCOS and Graves’
diseases may sometimes be too mild to be recognized by patients. Thus,
the underrepresentation of the identified population may occur and
eclipse the accuracy of prevalence.