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Low Birth Weight Is Not Associated with Thyroid Autoimmunity: A Population-Based Twin Study

Department of Endocrinology and Metabolism (T.H.B., P.S.H., A.B.R., J.B.H., L.H.), Odense University Hospital, 5000 Odense C, Denmark; and The Danish Twin Registry (P.S.H., A.S., K.O.K.), University of Southern Denmark, 5230 Odense M, Denmark

Address all correspondence and requests for reprints to: Thomas Heiberg Brix, M.D., Ph.D., Department of Endocrinology and Metabolism, Odense University Hospital, Sdr. Boulevard 29, 5000 Odense C, Denmark. E-mail: thomas.brix{at}ouh.fyns-amt.dk.

Context: Low birth weight has been proposed as a risk factor for the development of antibodies toward thyroid peroxidase (TPOAb) and thyroglobulin (TgAb) in adult life. However, the association could also be due to genetic or environmental factors affecting both birth weight and the development of thyroid autoantibodies. The effect of these confounders can be minimized through investigation of twin pairs.

Objective and Design: To examine the impact of low birth weight on the development of thyroid autoimmunity, we studied whether within-twin-cohort and within-twin-pair differences in birth weight are associated with differences in the serum concentration of TPOAb and TgAb in adult life.

Participants: We studied 1024 euthyroid twin individuals who were distributed in 512 same-sex twin pairs.

Methods: Original midwife protocols were traced manually through the Provincial Archives of Denmark. TPOAb and TgAb were measured using solid-phase time-resolved fluoroimmunometric assays.

Results: There were no statistically significant associations between birth weight and serum concentrations of TPOAb [regression coefficient (ß) = 0.003 (95% confidence interval, –0.010 to 0.015); P = 0.67] or TgAb [ß = 0.002 (–0.010 to 0.014); P = 0.77]. When restricting the analysis to twin pairs with a within-pair difference in birth weight of 500 g or greater or to twin pairs born 4 wk or more before term, the regression coefficients were almost unchanged. Controlling for potential confounders (sex, zygosity, gestational age, TSH, and smoking) did not change the findings of nonsignificant regression coefficients.

Conclusion: Low birth weight per se has no evident role in the etiology of thyroid autoimmunity.