Low birth weight is not associated with clinically overt thyroid disease:: a population based twin case-control study

OBJECTIVE In recent years low birth weight has been proposed as a risk factor for the development of several chronic diseases in adult life, including diabetes and subclinical autoimmune thyroid disease. The association could, however, also be due to genetic or environmental factors affecting both birth weight and adverse health outcomes in adult life. Moreover, it is at present unknown whether or not low birth weight is associated with an increased risk of developing clinically overt thyroid disease. The aim of the present study was to investigate the impact of birth weight and several other birth characteristics on the development of clinically overt thyroid disease. DESIGN A twin case-control study of same sex twin pairs. PATIENTS One hundred and thirty-one same sex twin pairs (262 twin individuals) discordant for clinically overt thyroid disease, ascertained from a population based nation-wide twin register. MEASUREMENTS Information about birth weight, birth length, birth order (first vs. second born), and prematurity was obtained from the original midwife records. RESULTS Forty-nine twin pairs were discordant for clinically overt autoimmune thyroid disease (Graves' disease = 35 and Hashimoto's thyroiditis = 14) and 82 pairs were discordant for overt nonautoimmune thyroid disease (Simple goitre = 79 and toxic nodular goitre=3). Overall, there was no difference in birth weights between probands and the healthy co-twins in monozygotic (MZ, n = 39) or in dizygotic (DZ, n = 92) pairs (MZ: mean +/- SE: 2619 +/- 93 g vs. 2553 +/- 89 g, P = 0.40; DZ: 2576 +/- 45 g vs. 2585 +/- 49, P = 0.86). By means of logistic regression, the impact of other birth characteristics such as birth length, birth order (first vs. second born), and prematurity was tested. None of the variables reached statistical significance. Subdividing the twin pairs into those discordant for clinically overt Graves' disease, Hashimoto's thyroiditis, and nonautoimmune thyroid disease did not change the results. CONCLUSIONS This is the first study of the effect of birth weight and other birth characteristics on the subsequent development of clinically overt thyroid disease in which maternal, socioeconomic, and to a high degree, genetic factors have been controlled for. Our study did not show any effect of birth weight or any of the other birth characteristics on the risk of developing clinically overt autoimmune or nonautoimmune thyroid disease.