Does response bias influence population studies of thyroid disorders?

Association of socioeconomic status with iodine supply and thyroid disorders in northeast Germany

OBJECTIVES

Studies on the potential association of socioeconomic status with iodine supply and the risk for thyroid disorders from developed countries are sparse. Socioeconomic status, however, may particularly impact the efficiency of iodine prophylaxis programs, which are based on the voluntary principle. This study aims to investigate whether the socioeconomic status is cross-sectionally and longitudinally related to low urinary excretion or thyroid disorders in the population of northeast Germany.

METHODS

Data of the population-based Study of Health in Pomerania were used. The study population comprised 4056 adults for cross-sectional and 2860 adults for longitudinal analyses. Assessment of socioeconomic status comprised different scales of education, income, employment, and occupation. Thyroid-related outcomes included urinary iodine excretion, serum thyrotropin, and sonographically defined goiter and nodules. Statistical analyses were adjusted for confounders.

RESULTS

Some of the socioeconomic variables were associated with thyroid-related characteristics in cross-sectional analyses. For example, there was an overall tendency for groups with higher education and higher income to have larger thyroid volumes and an increased risk of goiter. However, most of these associations did not attain statistical significance after correcting the target p-value for multiple testing. Longitudinal analyses did not demonstrate consistent results.

CONCLUSIONS

Socioeconomic status neither substantially influences iodine supply nor does it have a major impact on the prevalence and incidence of thyroid deficiency-related disorders in the adult population of northeast Germany, indicating a good efficacy of the German iodine fortification program in all socioeconomic groups.

[Study of Health in Pomerania (SHIP). Concept, design and selected results]

The Study of Health in Pomerania (SHIP) is a population-based project conducted in Northeast Germany. The main purpose is to assess prevalence and incidence of common risk factors, subclinical disorders and clinical diseases, and to investigate associations and interactions among them. The first SHIP cohort included 4,308 individuals at baseline (SHIP-0, response 68.8%) and 3,300 after 5 years (SHIP-1, response 83.6%). Currently, data are being collected for the 11-year follow-up (SHIP-2). In parallel, baseline examinations of a second, independent cohort (SHIP-TREND) are being conducted. The assessments range from interviews to laboratory analyses; somatometric and blood pressure measurements; dental, dermatological, cardio-metabolic and various ultrasound examinations; to more demanding methods such as cardiopulmonary exercise tests, sleep monitoring and whole-body magnetic resonance imaging. Analyses of baseline SHIP-0 data revealed valuable information on the health status of the West Pomeranian population, and generally confirmed the hypothesis of an accumulation of common risk factors and diseases in this region. The population-based character of the study and the profound information on potential exclusion criteria allow for analyses of population-representative reference values. The comprehensive data set offers various opportunities for association studies to gain new insight into the complex relations among risk factors, subclinical and clinical disease parameters. SHIP is an established population-based project conducted in Northeast Germany with a very comprehensive characterization of study subjects. After the data collection for SHIP-2 and SHIP-TREND is finished, a comprehensive data set will be available that describes complex interactions of health determinants.

Five-year change in morphological and functional alterations of the thyroid gland: the Study of Health in Pomerania

BACKGROUND

While cross-sectional data on thyroid epidemiology are common, few longitudinal data on the incidence and progression of thyroid disorders exist. The aim of this study was to analyze the 5-year change and the incidence of morphological and functional alterations of the thyroid gland in a previously iodine-deficient area.

METHODS

We used 5-year follow-up data based on 2941 participants (20-79 years) of the population-based Study of Health in Pomerania. The thyroid structure and size were measured by ultrasonography. Thyroid function and serum autoantibodies to thyroperoxidase (anti-TPO-Ab) were evaluated.

RESULTS

During follow-up, the median iodine/creatinine ratio decreased slightly from 134.2 to 130.4 μg/g. The mean thyroid volume was nonsignificantly increased by 0.73 mL [confidence interval (CI) -1.68-3.41 mL]. The incidence rate of goiter was 34.0 [CI 30.7-37.7] per 1000 person-years (py), the incident rate of nodules was 16.6 [CI 14.5-19.1] per 1000 py. Median serum thyrotropin levels increased slightly (0.10 [CI -0.10-0.34 mIU/L]). The incidence of positive anti-TPO-Ab was 3.4 [CI 2.6-4.7] per 1000 py. For all thyroid parameters, the regression rates were higher than the incidence rates.

CONCLUSIONS

After iodine fortification became effective in this previously iodine-deficient region, there is a negative net balance between occurring and reverting goiter and nodules. Along with balanced incidences of hyper- and hypothyroid dysfunction and the low incidence of autoimmune-related findings, these findings indicate that the current iodine status is sufficient to prevent iodine deficiency-related disorders in the adult population of the study region.

Partial misspecification of survey design features sufficed to severely bias estimates of health-related outcomes

OBJECTIVE

Surveys frequently deviate from simple random sampling through the use of unequal probability sampling, stratified sampling, and multistage sampling. This work uses a survey of public health to systematically illustrate the effects of incompletely accounting for strata, clustering, and weights.

STUDY DESIGN AND SETTING

Data analysis was based on the Study of Health in Pomerania (n=4,308, 20-79 years), a two-stage regional survey with high sampling fractions at the first stage. Effects of survey design features comprising weights, stratification, clustering, and finite population correction on point and variance estimates of lifestyle indicators and clinical parameters were assessed.

RESULTS

Misspecifications of the survey design substantially affected both the point estimates of health characteristics and their standard errors (SEs). The strongest bias in SEs concerned the omission of the second sampling stage. Ignoring the sampling design led to minor differences in variance estimates from the complete setup. Weighting predominantly affected point estimates of lifestyle factors.

CONCLUSION

A partial misspecification of survey design elements may bias variance estimates severely and is sometimes even more harmful compared with completely neglecting design elements. If subgroups are sampled at different rates, weighting is of particular relevance with regard to prevalence estimates of lifestyle indicators.