Low birth weight is not associated with thyroid autoimmunity: a population-based twin study

The co-occurrence and cumulative prevalence of hypertension, rheumatoid arthritis, and hypothyroidism in preterm-born women in the Women's Health Initiative

Emerging evidence suggests that preterm-born individuals (<37 weeks gestation) are at increased risk of developing chronic health conditions in adulthood. This study compared the prevalence, co-occurrence, and cumulative prevalence of three female predominant chronic health conditions - hypertension, rheumatoid arthritis [RA], and hypothyroidism - alone and concurrently. Of 82,514 U.S. women aged 50-79 years enrolled in the Women's Health Initiative, 2,303 self-reported being born preterm. Logistic regression was used to analyze the prevalence of each condition at enrollment with birth status (preterm, full term). Multinomial logistic regression models analyzed the association between birth status and each condition alone and concurrently. Outcome variables using the 3 conditions were created to give 8 categories ranging from no disease, each condition alone, two-way combinations, to having all three conditions. The models adjusted for age, race/ethnicity, and sociodemographic, lifestyle, and other health-related risk factors. Women born preterm were significantly more likely to have any one or a combination of the selected conditions. In fully adjusted models for individual conditions, the adjusted odds ratios (aORs) were 1.14 (95% CI, 1.04, 1.26) for hypertension, 1.28 (1.12, 1.47) for RA, and 1.12 (1.01, 1.24) for hypothyroidism. Hypothyroidism and RA were the strongest coexisting conditions [aOR 1.69, 95% CI (1.14, 2.51)], followed by hypertension and RA [aOR 1.48, 95% CI (1.20, 1.82)]. The aOR for all three conditions was 1.69 (1.22, 2.35). Perinatal history is pertinent across the life course. Preventive measures and early identification of risk factors and disease in preterm-born individuals are essential to mitigating adverse health outcomes in adulthood.

Birthweight and subsequent risk for thyroid and autoimmune conditions in postmenopausal women

The objective of this study was to determine the association between birthweight and risk of thyroid and autoimmune conditions in a large sample of postmenopausal women. Baseline data from the Women's Health Initiative (n = 80,806) were used to examine the associations between birthweight category (<6 lbs., 6-7 lbs. 15 oz, 8-9 lbs. 15 oz, and ≥10 lbs.) and prevalent thyroid (underactive and overactive thyroid and goiter) and autoimmune (lupus, rheumatoid arthritis (RA), multiple sclerosis, ulcerative colitis/Crohn's disease) conditions. Follow-up questionnaire data were used to examine the associations between birthweight and incident underactive and overactive thyroid, lupus, and RA. Logistic and Cox proportional hazards regression models were used to estimate crude and adjusted odds (OR) and hazards ratios (HR), respectively. Overall, women born weighing ≥10 lbs. had an increased risk for underactive thyroid [OR 1.14 (95% CI 1.02, 1.28)] and incident lupus [HR 1.51 (95% CI 1.12, 2.03)] and a decreased risk for overactive thyroid [OR 0.67 (95% CI 0.50, 0.92)] compared to women born weighing 6-7.99 lbs., after adjustment for adult BMI, demographic variables, and lifestyle factors. Further, women born weighing <6 lbs. were at increased risk for underactive thyroid [OR 1.13 (95% CI 1.04, 1.22)]. Birthweight was not associated with other thyroid or autoimmune disorders. High birthweight was associated with later-life thyroid and autoimmune conditions while low birthweight was associated with underactive thyroid. Preconception and prenatal interventions aimed at reducing the risk of both high and low birthweights may reduce the burden of later-life thyroid and autoimmune conditions.

Shared etiology of type 1 diabetes and Hashimoto's thyroiditis: a population-based twin study

OBJECTIVE

Type 1 diabetes and Hashimoto's thyroiditis frequently cluster in individuals and in families, indicating shared origins. The objective of this study was to investigate familial co-aggregation of these diseases and to quantify shared genetic and environmental factors.

DESIGN

This study is a twin cohort study.

METHODS

National health registers were used to identify cases among 110 814 Swedish twins. Co-aggregation was calculated as risk ratios for type 1 diabetes among co-twins of individuals with Hashimoto's thyroiditis, and vice-versa. Variance explained by genetics (i.e. heritability), and the proportions thereof shared between the diseases, was estimated by contrasting associations in monozygotic and dizygotic twins using structural equation models.

RESULTS

Individuals with one disease were at a high risk for the other disease (adjusted risk ratio: 11.4 (95% CI: 8.5-15.3)). Co-aggregation was more common in monozygotic than in dizygotic pairs, with adjusted risk ratios of 7.0 (95% CI: 3.2-15.1) and 1.7 (95% CI: 0.7-4.1), respectively. Genetic effects shared across diseases accounted for 11% of the variance for type 1 diabetes and 9% of the variance for Hashimoto's thyroiditis, while environmental factors unique to individual twins, but shared across diseases, accounted for 10% of the variance for type 1 diabetes and 18% of the variance for Hashimoto's thyroiditis.

CONCLUSIONS

Both genes and environment unique to individual twins contribute to considerable etiologic overlap between type 1 diabetes and Hashimoto's thyroiditis. These findings add to the current knowledge on the mechanisms behind autoimmune disease clustering and could guide future research aimed at identifying pathophysiological mechanisms and intervention targets.

Thyroid Function in Monozygotic Twins with Intra-twin Birth Weight Differences: A Prospective Longitudinal Cohort Study

OBJECTIVE

To analyze the long-term impact of birth weight (BW) on thyroid function in genetically identical twins with intra-twin BW differences from birth to adolescence.

STUDY DESIGN

In total, 52 monozygotic twin pairs underwent at least one analysis of thyroid function at mean ages of 10.1 years (27 pairs), 15.1 years (35 pairs), and 17.4 years (36 pairs); 18 pairs donated blood at all time points. BW difference of <1 SDS was defined as concordant, BW difference ≥1 SDS as discordant.

RESULTS

In concordant twins, no significant differences were observed. In the discordant group, smaller twins had higher mean thyroid-stimulating hormone (TSH) than their larger co-twins at 10.1 years (3.6 vs 2.5 μU/mL; P = .04) and 15.1 years (2.6 vs 2.2 μU/mL; P = .08). Smaller twins showed lower mean thyroxine than larger co-twins at 10.1 years (7.8 vs 8.2 μg/dL P = .05) and 17.4 years (7.7 vs 8.4 μg/dL; P = .03), and a tendency at 15.1 years (6.9 vs 7.4 μg/dL; P = .09). Calculation of TSH-thyroxine ratio revealed significant differences in the discordant group, with greater ratios in the smaller twin at 10.1 years (0.5 vs 0.3; P = .006) and 15.1 years (0.4 vs 0.3; P = .04).

CONCLUSIONS

In this group of monozygotic twins with intra-twin BW differences, BW seemed to exert a long-lasting impact on thyroid function. This may be due to a delay in hypothalamic-pituitary-thyroid axis maturation, with TSH resistance during childhood and early adolescence in children with low BW.

Are Perinatal Events Risk Factors for Childhood Thyroid Autoimmunity?

BACKGROUND

Environmental and genetic factors possibly trigger thyroid autoimmunity. Studies on perinatal risk factors for childhood thyroid autoimmunity are sparse.

OBJECTIVES

The aim was to investigate if perinatal factors, family history of autoimmune diseases, and HLA-DQ genotypes contribute to thyroid autoimmunity in the Diabetes Prediction in Skåne (DiPiS) study.

METHODS

Samples from 1,874 ten-year-old children were analyzed for autoantibodies to thyroid peroxidase (TPOAb), thyroglobulin (TGAb), and HLA-DQ genotypes. Information on perinatal events and family history of autoimmunity was gathered prospectively in questionnaires.

RESULTS

Thyroid autoimmunity was found in 6.9% of the children (TPOAb 4.4%, TGAb 5.8%, both autoantibodies 3.3%) and was overrepresented in girls. Prematurity was positively related to TGAb (OR: 2.4, p = 0.003, p_c = 0.021). Autoimmune diseases in the family increased the risk of thyroid autoimmunity: TPOAb (OR: 2.2, p = 0.012), any autoantibody (OR: 1.7, p = 0.04), and both autoantibodies (OR: 2.2, p = 0.024). A first-degree relative (FDR) with thyroid disease increased the risk for TPOAb (OR: 2.4, p = 0.03) and both autoantibodies (OR: 2.6, p = 0.03), a FDR or sibling with celiac disease increased the risk for both autoantibodies (OR: 3.7, p = 0.03, and OR: 4.8, p = 0.003), a FDR or sibling with diabetes increased the risk for thyroid autoantibody (OR: 3.0, p = 0.01, and OR: 5.4, p = 0.032), and a father with rheumatic disease increased the risk for TPOAb (OR: 15.2, p = 0.017), TGAb (OR: 11.3, p = 0.029), any autoantibody (OR: 9.6, p = 0.038), and both autoantibodies (OR: 20, p = 0.01).

CONCLUSIONS

Thyroid autoimmunity was found in 6.9% of the 10-year-old children who were being followed for their risk of type 1 diabetes. No relation to perinatal factors was found, with the exception of a possible association between prematurity and TGAb. Family history of autoimmune diseases increased the risk of thyroid autoimmunity.

MECHANISMS IN ENDOCRINOLOGY: Neurodevelopmental disorders in children born to mothers with thyroid dysfunction: evidence of fetal programming?

Fetal programming is a long-standing, but still evolving, concept that links exposures during pregnancy to the later development of disease in the offspring. A fetal programming effect has been considered within different endocrine axes and in relation to different maternal endocrine diseases. In this critical review, we describe and discuss the hypothesis of fetal programming by maternal thyroid dysfunction in the context of fetal brain development and neurodevelopmental disorders in the offspring. Thyroid hormones are important regulators of early brain development, and evidence from experimental and observational human studies have demonstrated structural and functional abnormalities in the brain caused by the lack or excess of thyroid hormone during fetal brain development. The hypothesis that such abnormalities introduced during early fetal brain development increase susceptibility for the later onset of neurodevelopmental disorders in the offspring is biologically plausible. However, epidemiological studies on the association between maternal thyroid dysfunction and long-term child outcomes are observational in design, and are challenged by important methodological aspects.

Regulation of the pituitary-thyroid axis in adulthood is not related to birth weight: evidence from extremely birth weight-discordant monozygotic Danish twin pairs

BACKGROUND

Low birth weight has been linked with changes in thyroid function in adulthood, but it is unknown whether fetal programming or underlying genetic and environmental factors explains the association. We hypothesized that birth weight influences the pituitary-thyroid set point in adults.

METHODS

A total of 152 birth weight-discordant monozygotic twin pairs with a median age of 57 years (interquartile range: 33-63) were ascertained from the Danish Twin Registry in 2010. Serum thyroid-stimulating hormone (TSH), free thyroxine (FT4), and triiodothyronine (T3) levels were measured. Birth weights were retrieved from midwife records (individuals born before 1973) and the Danish Birth Record Registry (all other participants)

RESULTS

Birth weight was inversely associated with serum levels of FT4 (β=-0.48 pmol/[L·kg], p=0.014) and serum T3 (β=-0.09 nmol/[L·kg], p=0.010), but not serum TSH after adjustment for age, sex, and current use of tobacco products, when the twins were investigated as singletons. Serum levels of TSH and T3 were similar in within twin-pair analyses, while serum FT4 was higher in twins with the lowest birth weight (median difference 0.3 mIU/L). When the analyses were repeated in twin pairs (n=46 pairs) characterized by extreme difference in birth weight (>0.5 kg), serum TSH, T3, and FT4 levels were similar in twins with high and low birth weight. The proportion of individuals with serum TSH level >4 mIU/L or <0.3 mIU/L was identical in both groups.

CONCLUSIONS

No overall evidence of an association between birth weight and adult pituitary-thyroid axis set point, after control for genetic and environmental factors, could be demonstrated.

Childhood weight gain and thyroid autoimmunity at age 60-64 years: the 1946 British birth cohort study

BACKGROUND

Complex bidirectional relationships have been described between body weight, thyroid function, and risk of thyroid disorders, including thyroid autoimmunity. We used a life-course approach to examine the potential association of childhood or adult body weight with the risk of thyroid autoimmunity and other thyroid disorders at age 60-64 years in a large population-based birth cohort study.

METHODS

In the UK Medical Research Council 1946 British Birth Cohort study, at age 60-64 years, 1277 women and 1185 men (78% of the target sample) responded to a postal questionnaire, which included questions on thyroid disease and thyroid medication. Circulating antithyroid peroxidase antibodies, free T4, and TSH concentrations were measured in 1057 women and 997 men at a subsequent clinic visit. Birth weight was recorded, and height and weight were measured at ages 2, 4, 6, 7, 11, 15 years and also repeatedly in adulthood.

RESULTS

At age 60-64 years, 10.9% of women (139 of 1277) and 2.3% of men (27 of 1185) reported they were taking T4, and 11.5% of women (122 of 1057) and 3.3% of men (33 of 997) had positive anti-TPO antibodies (>100 IU/mL), consistent with thyroid autoimmunity. Among women, both T4 use and positive anti-TPO antibodies at age 60-64 years were positively associated with childhood body weight, childhood overweight, and adult body mass index. Childhood weight gain between 0 and 14 years of age was positively associated with later T4 use (odds ratio 1.21, 95% confidence interval 1.03-1.42) and positive anti-TPO antibodies (1.21, 1.00-1.47). Women who were overweight or obese at age 14 years (127 of 972) had a higher risk of later positive anti-TPO antibodies (2.05, 1.12-3.76). In men and women without any thyroid disorders, serum free T4 concentrations were inversely associated with concurrent body mass index (P = .002).

CONCLUSIONS

Childhood weight gain and childhood overweight conferred an increased susceptibility to later hypothyroidism and thyroid autoimmunity, particularly in women.

Twin studies as a model for exploring the aetiology of autoimmune thyroid disease

Twins are an important resource for evaluating the relative contribution of genetic and environmental factors in determining a phenotype. During the last decades, a number of twin studies have investigated the aetiology of several phenotypes related to thyroid autoimmunity. Taken together, these studies have provided valid and unbiased information regarding the influence of genetic and environmental factors in the aetiology of autoimmune thyroid disease (AITD). The comparison of concordance rates between monozygotic (MZ) and dizygotic twins provides irrefutable evidence of a genetic component, and biometric twin modelling shows that approximately 75% of the total phenotypic variance in AITD is because of genetic effects. On the other hand, the lack of complete concordance in MZ twin pairs is proof of environmental and/or epigenetic factors also playing an important role. The impact of environmental triggers such as cigarette smoking, birth characteristics, infection with Yersinia enterocolitica, microchimerism and degree of X chromosome inactivation (XCI) has been evaluated by investigating AITD discordant twin pairs. These studies indicate that smoking, Y. enterocolitica infection and skewed XCI may be causally associated with clinically overt AITD, but not with the presence of thyroid autoantibodies in euthyroid subjects. Microchimerism, but not birth weight, might play a role in AITD. Twin studies offer several features that uniquely enhance our ability to localize genes and understand their function. Future twin studies should incorporate information on genetic, epigenetic and environmental variation thereby enhancing our ability to quantify the precise effect of specific risk factors.

Preterm birth and risk of medically treated hypothyroidism in young adulthood

OBJECTIVE

Previous studies suggest that low birth weight is associated with thyroid autoimmunity and hypothyroidism in later life, but the potential effect of preterm birth, independent of foetal growth, is unknown. Our objective was to determine whether preterm birth is independently associated with medically treated hypothyroidism in young adulthood.

DESIGN/PARTICIPANTS

National cohort study of 629,806 individuals born in Sweden from 1973 through 1979, including 27,935 born preterm (<37 weeks).

MEASUREMENTS

Thyroid hormone prescription during 2005-2009 (ages 25·5-37·0 years), obtained from all outpatient and inpatient pharmacies throughout Sweden.

RESULTS

Preterm birth was associated with increased relative odds of thyroid hormone prescription in young adulthood, after adjusting for foetal growth and other potential confounders. This association appeared stronger among twins than singletons (P = 0·04 for the interaction). Twins had increased relative odds across the full range of preterm gestational ages, whereas singletons had increased relative odds only if born very preterm (23-31 weeks). Among twins and singletons, respectively, adjusted odds ratios for individuals born preterm (<37 weeks) were 1·54 (95% CI, 1·11-2·14) and 1·08 (95% CI, 0·98-1·19), and for individuals born very preterm (23-31 weeks) were 2·62 (95% CI, 1·30-5·27) and 1·59 (95% CI, 1·18-2·14), relative to full-term births.

CONCLUSIONS

This national cohort study suggests that preterm birth is associated with an increased risk of medically treated hypothyroidism in young adulthood. This association was independent of foetal growth and appeared stronger among twins than singletons. Additional studies are needed to confirm these new findings in other populations and to elucidate the mechanisms.

Determinants of thyroid autoantibody production in Hashimoto's thyroiditis

Hashimoto's thyroiditis (HT) is the most prevalent thyroid autoimmune disorder, characterized by the presence of specific thyroid autoantibodies (TAb). The development of autoimmunity, including TAb production and clinical presentation of HT, is determined by a complex interplay of genetic susceptibility and several endogenous and environmental factors, which are discussed in this article. During the progression of the disease, TAb production precedes clinical manifestations, although the correlation between TAb concentrations and thyroid function is weak. We do not treat euthyroid HT patients despite elevated TAb; while in hypothyroidism, replacement therapy with l-thyroxine is required. Until now, an effective approach to prevent TAb production and the development of clinical disease has not yet been established. However, further identification of risk factors and their interaction may help in the prevention of thyroid autoimmunity.

Aggregation of thyroid autoantibodies in twins from opposite-sex pairs suggests that microchimerism may play a role in the early stages of thyroid autoimmunity

CONTEXT

Microchimerism is the presence of small populations of cells from one individual in another genetically distinct individual. This phenomenon can arise from pregnancy, blood transfusion, or bidirectional cell trafficking between twins in utero. Microchimerism has recently been proposed to play a role in the pathogenesis of thyroid autoimmunity. In that case, twins from opposite-sex pairs (OS) should have an increased risk of thyroid autoantibodies (TA).

AIM

The aim of the study was to compare the frequency of TA in twin individuals from OS and monozygotic (MZ) twin pairs.

DESIGN

This was a case-control study of 240 individuals (120 females and 120 males) from OS twin pairs (cases) and 568 control individuals from MZ pairs (284 females and 284 males).

METHODS

Antibodies toward thyroid peroxidase (TPOAb), thyroglobulin (TgAb), and the TSH receptor (TSHRAb) were measured and considered positive if greater than 60 U/ml, greater than 60 U/ml, and greater than 1.0 U/liter, respectively.

RESULTS

The frequency of TPOAb, TgAb, and TSHRAb among female cases was 15.0, 5.0, and 4.2%, respectively, which was higher than the corresponding prevalences in the female control population: 7.4% (P = 0.018), 1.1% (P = 0.023), and 0.7% (P = 0.026), respectively. However, when corrected for the number of phenotypes studied (TPOAb, TgAb, TSHRAb, and any thyroid antibody), the association remained significant only in the combined group, P(corrected) = 0.012. Essentially similar results were obtained in males.

CONCLUSION

Both female and male twins from OS pairs, as opposed to MZ pairs, have an increased frequency of TA, indicating a potential role of microchimerism in developing TA.

Epidemiology and health services: year in review

PURPOSE OF REVIEW

Epidemiology began as the study of the occurrence of disease in populations. Initial studies focused on the presence of diseases by person, place, and time to identify risk factors for disease. The field has grown to include many subfields recognizing the contributions of multiple factors to disease susceptibility.

RECENT FINDINGS

This review will focus on articles published October 2005-November 2006 relevant to genetic epidemiology, pharmacoepidemiology, and social determinants of health and health disparities.

SUMMARY

Genetic epidemiology is the study of genetic contributions to disease occurrence, and of population-based genetic risks for disease. Genetics influences not only susceptibility to disease, but also the phenotype and severity of disease. Pharmacoepidemiology examines the outcomes of medication use, including clinical benefits and adverse effects. Gene-environment interactions are also included in this area. Typical topics in pharmacoepidemiology include studies of medication use, risks, benefits and genetic factors influencing the metabolism of medications. Social epidemiology is the study of social determinants of health, including psychosocial behaviors, socioeconomic status, interactions of individuals, the collective health of the community in which a person lives, and the position of the individual within society. Social epidemiology is directly relevant to studies of socioeconomic and ethnic disparities.