Age-specific thyrotropin references decrease over-diagnosis of hypothyroidism in elderly patients in iodine-excessive areas.

Abstract

Acute or chronic exposure to excess iodine has detrimental effects on thyroid physiology; therefore, this study aimed to determine the prevalence of overt hypothyroidism (OH) and subclinical hypothyroidism (SCH) in an elderly population residing in geographical areas with chronic exposure to excess iodine intake and to analyse contributing risk factors. This cross-sectional study was conducted from 2016 to 2017 in areas of Jiangsu Province that have documented chronic exposure to high iodine intake. We enroled 2559 adult participants using a multistage, stratified sampling method. Urinary iodine concentration (UIC), serum thyroid-stimulating hormone (TSH) level and other relevant parameters were measured. Demographic information was recorded using a standardized questionnaire. The age-specific TSH references were determined by the National Academy of Clinical Biochemistry guidelines. Univariate and multivariate logistic regression analyses were performed to identify risk factors for hypothyroidism in the study population. The median UIC of participants was 307.3 µg/L (interquartile range: 200.7, 469.8 μg/L). The prevalence of OH in subjects ≥70 years using laboratory reference ranges was 2.37%; however, it decreased to 1.78% with the use of an age-specific reference range. Similarly, the prevalence of SCH also declined drastically from 29.59% to 2.96% with the application of an age-specific reference range. In both univariate and multivariate models, advanced age, female gender and high UIC were identified as risk factors for hypothyroidism. Usage of age-specific TSH reference ranges led to a significantly lower prevalence of OH and SCH in the study population, thus preventing unnecessary over-diagnosis and over-treatment.