474 THYROID Volume 15, Number 5, 2005 © Mary Ann Liebert, Inc. Sequential Studies on Thyroid Antibodies During Pregnancy P.P.A. Smyth, 1 C.N. Wijeyaratne, 3 W.N. Kaluarachi, 3 D.F. Smith, 1 L.D.K.E. Premawardhana, 2 A.B. Parkes, 2 A. Jayasinghe, 3 D.G.H. de Silva, 3 and J.H. Lazarus 2 Thyroid antibodies were measured sequentially in 25 pregnant women from a Sri Lankan population. A high prevalence of antithyroid antibodies, particularly antithyroglobulin antibodies (TgAb) had previously been demonstrated in female schoolchildren drawn from this population. In the present study TgAb were detected in 36.8% of nonpregnant controls while thyroid peroxidase antibody (TPOAb) positivity was present in 26.3%. The prevalence of both antibodies in the pregnancy study group showed a progressive decline compared to nonpregnant controls throughout gestation becoming undetectable in the third trimester. The results are con- sistent with an immunosuppressive effect of pregnancy in a population in whom high thyroid autoantibody titers may have resulted from a recent salt iodization program. Introduction T HE PREGNANT STATE is associated with many physiologic changes in thyroid function (1). However, other alter- ations such as changes in thyroid volume, iodine uptake, uri- nary iodine excretion, and thyroid autoantibody titers, may represent compensatory responses to environmental or hu- moral factors. Increased thyroid volume during normal preg- nancy may be related to the dietary iodine intake of the study population (1,2). Previous studies have demonstrated that the known immunosuppressive effect of pregnancy on thy- roid function (3) results in a decline in thyroid autoantibody titers including thyroglobulin antibodies (TgAb), thyroid peroxidase antibodies (TPOAb), and thyrotropin receptor antibodies (TRAb; 4,5). Iodine-induced autoimmunity in terms of positive TgAb has been attributed to the increased antigenicity of more highly iodinated thyroglobulin (6–9). In addition, administration of iodine to patients with preexist- ing iodine deficiency has been reported to induce thyroid autoimmunity (10) with increasing levels of both TPOAb and TgAb (11,12). Our group has previously demonstrated a high level of thyroid antibody positivity particularly thyroglobu- lin antibody TgAb (14.3%–69.7% prevalence) in Sri Lankan schoolgirls (13). The precise cause of TgAb positivity in such subjects is unclear but it may be a consequence of the salt iodization programme introduced in Sri Lanka in 1993. The objective of the present study was to demonstrate if the high titers of thyroid antibodies previously reported in Sri Lankan schoolgirls in 1998 were also present in adult females and to determine alterations in antibody titer during normal preg- nancy and its possible relationship to dietary iodide intake. Subjects and Methods Subjects’ pregnancies Twenty-five pregnant Sri Lankan women aged 20 to 44; Median 32 years had sequential blood samples taken during each of the three trimesters of pregnancy and at 6 weeks post- partum. Mean gestation times  standard error (SE) in weeks were first trimester (9.8  0.23), second trimester (20.8  0.29), and third trimester (34.1  1.4). Spot urine samples were available from 19 of these subjects who were studied sequentially in first, second, and third trimesters. Nonpregnant controls Blood and urine samples were obtained from 57 non- pregnant female control subjects (aged 21–38; median, 28 years). Informed consent for the study was obtained in all cases. Methods Thyroid antibodies. TgAb (normal less than 98 kIU/L) and TPOAb (normal less than 19.4 kIU/L) were measured by en- zyme-linked immunosorbent assay (ELISA), as previously 1 Endocrine Laboratory, Department of Medicine and Therapeutics, Conway Institute of Biomolecular and Biomedical Research, Uni- versity College, Dublin, Ireland. 2 Department of Medicine, University of Wales College of Medicine Cardiff, United Kingdom. 3 Faculties of Medicine, Colombo, Peradeniya and Colombo North, Sri Lanka.