Primary hyperparathyroidism was diagnosed in 109 (2.1%) of 5202 women aged 55-75 years attending population based mammography screening.2
Diagnosis of primary hyperparathyroidism was based on repeated fasting serum calcium and intact serum parathyroid hormone concentrations and included women with normal serum calcium values.2
The disorder was verified histologically in 60 women who underwent parathyroidectomy. For each woman with primary hyperparathyroidism we randomly selected a control matched for age and season of biochemical investigation from the screened population (table ). Eligibility criteria for cases and controls included a serum creatinine concentration below 160 μmol/l to exclude the existence of severe renal diseases. All women gave informed consent for participation in the study, which was approved by the ethics committee.
Clinical characteristics of all women with hyperparathyroidism recruited by screening and of sample of women who had not retired from work 5 years before date of screening. Figures are means (SD)
All data on sick leave were provided by the regional social insurance office, which recorded sick leave and retirement within the screened population. In 48 (44%) case-control pairs neither the case nor the control had retired, because of age (mandatory retirement age 65 years) or illness, 5 years before the date of screening for primary hyperparathyroidism. The duration, cause, and type of sick leave for these case-control pairs was investigated in the 5 years before the day of screening. This period of time was the length of follow up within the study. Interview at biochemical diagnosis supported the absence of traditional symptoms of primary hyperparathyroidism,6
although three cases admitted to constantly feeling tired on direct questioning. Before screening the diagnosis was unknown to all affected women, although two of them had had hypercalcaemia.
Three cases and the same number of controls were housewives at the time of screening. The case or control of 27 pairs had retired because of disease (9 cases, 5 controls) or age (3 cases, 10 controls) during the 5 year period. Both the case and control were excluded from analysis from the date of any full time retirement in the pair to ensure equal years at risk for sick leave. Partial retirement (6 cases, 2 controls) was disregarded. Women with hyperparathyroidism who retired during the study had similar serum calcium and parathyroid hormone concentrations to those who completed the 5 year analysis. The total duration of follow up comprised 348 person years with an individual mean of 3.6 (SD 1.6) years.
For each woman total duration of sick leave was divided by the length of follow up within the study to standardise for variability among the case-control pairs. Benefits on full (100%) or half (50%) time and short (
1 week) or long (>1 week) time were available during the analysed time period. As these subtypes are partially exclusive recorded days for a particular sickness benefit were divided by the number of days at risk for this particular benefit type. A physician’s certificate for sick leave was required only for periods exceeding a week. The diagnoses on the certificates were grouped according to ICD-9 (international classification of diseases, ninth revision) and could be retrieved for 93% of the periods of long term sickness benefit.
We used the Wilcoxon non-parametric tests (Wilcoxon rank sum test and Wilcoxon signed rank test). The relation between the proportion of sick days and case or control status was also analysed on the basis of the logistic regression model estimated by the conditional maximum likelihood method. The variable sick day proportion was calculated as days of sick leave divided by the number of available days. It was considered both in continuous and categorised forms. The exact categorisation was found to be important. P<0.05 was considered significant. Descriptive results are presented as means (SD).