Validity of a thyrotoxicosis diagnosis code among women of fertile age.

Introduction: Graves' disease (GD) is the leading cause of hyperthyroidism in women of fertile age. Register-based studies rely on diagnostic codes; this study examined the validity of diagnosed thyrotoxicosis among women of fertile age in the Danish National Hospital Register (DNHR).

Methods: All women in the North Denmark Region aged 18-45 years with a diagnosis of thyrotoxicosis (Tenth International Classification of Diseases: E05.0-05.9) in the DNHR from 2017-2018 were identified and medical records reviewed.

Results: Among 263 women included, thyrotoxicosis was confirmed in 249 cases with a positive predictive value (PPV) of 94.7% (95% confidence interval (CI): 91.2-97.1%) for the diagnostic group (E05.0-05.9). Autoimmune hypothyroidism was the leading cause of misclassification. GD was the aetiology in 150 women, resulting in a PPV for GD of 57.0% (95% CI: 50.8-63.1%) in the diagnostic group (E05.0-05.9), increasing to 83.6% (95% CI: 76.4-89.3%) when restricted to women who were prescribed an antithyroid drug (ATD). The PPV for GD was highest when the specific diagnosis (DE05.0) was considered (PPV 91.1% (95% CI: 84.7-95.5%); however, 42 women with GD (28.0%) were first given a diagnosis of unspecified thyrotoxicosis (E05.9).

Conclusions: The validity of thyrotoxicosis identified in the DNHR among women of fertile age was high. Results emphasise the importance of how subtypes of thyrotoxicosis are defined and show that the combined use of diagnoses and prescriptions of ATD is warranted to define GD.

Funding: Novo Nordisk Foundation (grant no: NNF20OC0059465).

Trial registration: Not relevant.