July 2009

Editor:
Michael Bissell, MD, PhD, MPH

Iron deficiency and thyroid status in pregnancy
Assessing dietary iodine intake in the United States
Thyroid-stimulating hormone and left ventricular function
Role of Duffy antigen in modifying systemic and local tissue chemokine responses
Metabolomics and disorders of propionate metabolism

Iron deficiency and thyroid status in pregnancy

During the second and third trimester, pregnant women are highly vulnerable to iron deficiency anemia because their increased iron needs are rarely met by dietary sources. In industrialized countries, the prevalence of anemia and iron deficiency during pregnancy ranges from six to 28 percent and 24 to 44 percent, re-spectively. In developing countries, the majority of women are anemic in the second half of pregnancy. Requirements for thyroid hormone during pregnancy also sharply increase to maintain maternal euthyroidism and transfer thyroid hormone to the fetus. To support this, the iodine requirement in pregnancy increases from 150 to 250 µg/d, making maternal thyroid function particularly vulnerable in regions of mar-ginal iodine intake. Iron deficiency has multiple adverse effects on thyroid metabolism. It decreases circulating thyroid hormone concentrations, likely through impairment of the heme-dependent thyroid peroxidase (TPO) enzyme. It also blunts the efficacy of iodine prophylaxis, and iron repletion improves the efficacy of iodized salt in goitrous children with the condition. Two prospective studies, using two different measures of impaired thyroid function in pregnancy—an increased thyroid-stimulating hormone (TSH) level in the second trimester and hypothyroxinemia at 12 weeks’ gestation—reported that even mild maternal thyroid dysfunction may impair neurodevelopment in off-spring. Therefore, the authors conducted a study to investigate whether maternal iron status is a determinant of TSH or total T4 (TT4) concentrations, or both, during pregnancy in an area of borderline iodine deficiency. They collected samples of urine and blood in a representative national sample of pregnant Swiss women (n=365) in the second and third trimester of pregnancy. Data on maternal characteristics and supplement use were recorded. Concentrations of TSH, TT4, hemo-globin, mean corpuscular volume, serum ferritin, transferrin receptor, and urinary iodine were measured. Body iron stores were calculated and stepwise regressions performed to look for associations. The authors found that median urinary iodine was 139 µg/L (range, 30–433). In the third trimester, nearly 40 percent of women had negative body iron stores, 16 percent had a TT4 of less than 100 nmol/L, and six percent had a TSH of more than 4.0 mU/L. Compared with the women with positive body iron stores, the rel-ative risk of a TT4 of less than 100 nmol/L in the group with negative body iron stores was 7.8 (95 percent confi-dence interval, 4.1; 14.9). Of the 12 women with TSH of more than 4.0 mU/L, 10 had negative body iron stores. Serum ferritin, transferrin receptor, and body iron stores were highly significant predictors of TSH (standardized β, –0.506, 0.602, and –0.589, respectively; all P<.0001) and TT4 (standardized β, 0.679, –0.589, and 0.659, respectively; all P<.0001). The authors concluded that poor maternal iron status predicts higher TSH and lower TT4 concentrations during pregnancy in an area of borderline iodine deficiency.

Zimmermann MB, Burgi H, Hurrell RF. Iron deficiency predicts poor maternal thyroid status during pregnancy. J Clin Endocrinol Metab. 2007;92:3436–3440.

Correspondence: Dr. Michael B. Zimmermann at michael.zimmermann@ilw.agrl.ethz.ch

Assessing dietary iodine intake in the United States

Adequate dietary iodine intake is essential for producing the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Inadequate iodine intake can lead to the development of goiter and hypothyroidism. Since the iodization of salt in the 1920s, dietary iodine intake in the United States generally has been sufficient; however, iodine intake has varied over the years due to alterations in the iodine content of foods and the consumption of iodized salt. Iodine supplementation of salt and other foods has never been mandated, and iodine content of most foods is not listed on package labels. Reductions in U.S. dietary iodine since the 1970s have been variously ascribed to a possible reduction in the iodine content of dairy products, removal of iodate dough conditioners in commercially produced bread, new recommendations for reduced salt intake for blood pressure control, and increased use of noniodized salt in convenience foods. At the same time, it has been difficult to determine the primary sources of iodine in the current U.S. diet. The effects of cigarette smoking on dietary iodine intake and thyroid status too remain unclear. Cigarette smoke is converted to thiocyanates that act as potent inhibitors of iodine transport into the thyroid by competitively inhibiting the sodium/iodide symporter on the thyroid epithelial cell surface. The authors conducted a study in which they constructed a self-administered questionnaire to ascertain the types and quantities of iodine-containing foods and supplements in the diet, as well as smoking behavior, and to correlate the results of this questionnaire with a 24-hour food diary and urinary iodine and urinary cotinine excretion. The authors intended to validate this questionnaire for use in future studies of iodine nutrition in the United States, particularly among women of childbearing age. They assessed iodine intake in 46 healthy Boston-area adult volunteers. Questionnaire information was correlated with subjects’ 24-hour food diaries. Spot morning urine cotinine, a metabolite of nicotine from cigarette smoking, and iodine concentrations were measured. Ninety-one percent of 46 subjects were women (mean [±SD] age of 38 [±10] years). Information from 24-hour food diaries correlated well with questionnaire data. The median urinary iodine concentration (UIC) was 140 µg/L (range, 18–845 µg/L). Significant positive associations were noted between intake of yogurt (n=9; P=.01) and saltwater fish (n=6; P=.0003), and an inverse correlation was found between intake of bagels (n=5; P=.0006) and UIC. No associations were found between self-reported intake of milk or iodized salt, tobacco use, or urinary cotinine and UIC. The authors concluded that the questionnaire may not have been sufficiently comprehensive. Given the importance of adequate iodine intake, particularly among women of childbearing age, additional studies are warranted to determine important sources of dietary iodine in the United States.

Leung AM, Braverman LE, Pearce EN. A dietary iodine questionnaire: correlation with urinary iodine and food diaries. Thyroid. 2007;17:755–762.

Correspondence: Elizabeth N. Pearce at elizabeth.pearce@bmc.org

Thyroid-stimulating hormone and left ventricular function

A number of cardiovascular changes have been described in overt thyroid dysfunction, but adequate treatment has proven to be beneficial. However, whether to treat subclinical thyroid dysfunction, defined by normal levels of circulating free T4 and free T3 with thyroid-stimulating hormone (TSH) levels elevated or below the reference range, is more controversial. Demonstration of cardiovascular dysfunction, particularly diastolic dysfunction, in subjects with subclinical hypothyroidism as well as hyperthyroidism would favor early treatment of these conditions. However, most of these studies have included only a small number of subjects, and the re-sults are conflicting. Furthermore, even less data exist about cardiovascular function measured by echocardiography in relation to thyroid status in epidemiological studies. Several large population-based studies of cardiovascular diseases have been conducted in Tromso, Norway. In the fourth Tromso study, in 1994 and 1995, serum TSH was measured and conventional transthoracal echocardiography performed in more than 3,000 subjects. In the fifth Tromso study, in 2001, serum TSH was measured in nearly 8,000 subjects. From this co-hort, subjects with subclinical thyroid dysfunction and euthyroid cont-rols were examined using transtho-racal echocardiography, including pulsed-wave tissue Doppler imaging (PWTD) echocardiography. The authors conducted a cross-sectional epidemiolo-gical study and a nested case-control study at a university hospital to assess the relationship between serum TSH levels and cardiac function. A total of 2,035 subjects were included in the epidemiological study and 204 subjects in the nested case-control study (serum TSH of less than 0.50, 0.50–3.49, and 3.50–10.0 mIU/L in 20, 118, and 66 subjects, respectively, all with normal serum free T4 and free T3 levels). Left ventricular mass by body surface area (left ventricular mass index, LVMI) and indices of left ventricular function, as assessed by conventional and PWTD echocardiography, were recorded. No significant relationship was found between serum TSH levels and LVMI. In the nested case-control study, the subjects with serum TSH levels of 3.50 to 10.0 mIU/L had no signs of cardiac dysfunction. However, the PWTD data showed higher velocities at all measurement sites in the subjects with serum TSH of less than 0.50 mIU/L as compared with the euthyroid group. The authors concluded that, with the possible exception of overt hypo- and hyperthyroidism, there is no significant association between serum TSH levels and LVMI. Subjects with subclinical hypothyroidism, in whom the mean serum TSH levels are slightly above the reference range, appear to have normal cardiac function, whereas subjects with serum TSH levels of less than 0.5 mIU/L appear to have changes in myocardial velocities detected by PWTD.

Iqbal A, Schirmer H, Lunde P, et al. Thyroid stimulating hormone and left ventricular function. J Clin Endocrinol Metab. 2007;92:3504–3510.

Correspondence: Amjid Iqbal at amjid.iqbal@unn.no

Role of Duffy antigen in modifying systemic and local tissue chemokine responses

The observation that red blood cells bind the majority of CXCL8/IL-8 in whole blood led to the discovery that Duffy Ag, a minor blood group Ag, is a chemokine-binding protein. Historically known as the receptor for the malarial parasite Plasmodium vivax, Duffy Ag demonstrates high-affinity binding to CXCL8/IL-8 and other CXC and CC chemokines. The authors hypothesized that Duffy Ag functions as a chemokine reservoir and can regulate inflammatory responses by altering soluble chemokine concentrations in blood and local tissue compartments. To test this, they determined whether Duffy Ag “loss-of-function” pheno-types (human and murine) are associated with alterations in plasma chemokine concentrations during the innate inflammatory response to lipopolysaccharide stimulation (LPS). Plasma CXCL8 and CCL2 concentrations from humans homozygous for the GATA-l box polymorphism, a dfy polymorphism that abrogates erythrocyte chemokine binding, were higher than in heterozygotes following LPS stimulation of their whole blood in vitro. Similarly, dfy^-/- mice showed higher plasma MIP-2 concentrations than dfy^+/+ mice following LPS stimulation of whole blood in vitro. The authors then determined the relative contributions of erythrocyte and endo-thelial Duffy Ag in modifying che-mo-kine concentrations and neutrophil recruitment in the lungs following intratracheal LPS administration in dfy^-/- and dfy^+/+ mice reconstituted with dfy^-/- or dfy^+/+ marrow. They found that mice lacking endothelial dfy expression had higher MIP-2 and keratinocyte chemoattractant concentrations in their airspaces. Mice lacking erythrocyte dfy had higher MIP-2 and keratinocyte chemoattractant concentrations in their lung tissue vascular space but lower plasma chemokine concentrations associated with attenuated neutrophil recruitment into their airspaces. These findings indicate that dfy alters soluble chemokine concentrations in blood and local tissue compartments and enhances systemic bioavailability of chemokines produced during local tissue inflammation.

Lee JS, Wurfel MM, Matute-Bello G, et al. The Duffy antigen modifies systemic and local tissue chemokine responses following lipopolysaccharide stimulation. J Immunol. 2006;177:8086–8094.

Correspondence: Dr. Janet S. Lee at leejs3@upmc.edu

Metabolomics and disorders of propionate metabolism

Inborn errors of metabolism can have a severe impact on human health, so comprehensive diagnostic neonatal screening is used for early diagnosis to avoid poten-tially catastrophic physical and neurological effects. These defects can cause a buildup of toxic metabolites, resulting in serious disease or death at an early age. Neonatal screening with mass spectrometry is commonly used to test newborns for a wide array of inborn errors of metabolism using specific metabolites for diagnosis. The authors studied two diseases using metabolomics with nontargeted liquid chromatography-mass spectrometry (LC-MS) to simultaneously profile thousands of metabolites and obtain a more comprehensive metabolic profile of plasma. The power of untargeted metabolomics lies in its potential to broaden the medical profession’s understanding of disease biochemistry, identify new biomarkers, and improve disease categorization and treatment. Although originating in the disturbance of a single gene, inborn errors of metabolism produce highly diverse phenotypes and complex downstream metabolic effects, the result of the interplay of many biochemical path-ways and the individual’s interaction with the environment. The authors hypothesized that a metabolomics study may reveal additional differences between disease and normal plasma and, therefore, provide a more complete biochemical profile. They conducted a study that demonstrated the application of a new toolbox of methods to clinical chemistry, an approach with the potential to provide insights into the biochemical mechanisms of disease. They employed a screening platform that used untargeted, mass-based metabolomics of methanol-extracted plasma to find significantly different molecular features in human plasma samples from methylmalonic acidemia (MMA) and propionic acidemia (PA) patients and healthy individuals. Capillary reverse-phase liquid chromatography (4 µL/min.) was interfaced to a time-of-flight (TOF) mass spectrometer, and data were processed using nonlinear alignment software (XCMS) and an online database (METLIN) to find and identify metabolites differentially regulated in disease. The authors found that of the approximately 3,500 features measured, propionyl carnitine was easily identified as the best biomarker of disease (P value, 1.3 x 10^-18), demonstrating the proof-of-concept use of untargeted metabolomics in clinical chemistry discovery. Five additional acylcarnitine metabolites showed significant differentiation between plasma from patients and healthy individuals, and Υ-butyrobetaine was highly increased in a subset of patients. Two acylcarnitine metabolites and numerous unidentified species differentiate MMA and PA. Many metabolites that do not appear in any public database and that remain unidentified varied significantly between normal, MMA, and PA, underscoring the complex downstream metabolic effects resulting from the defect in a single enzyme. The authors concluded that this proof-of-concept study demonstrates that metabolomics can expand the range of metabolites associated with human disease and shows that this method may be useful for disease diagnosis and clinical patient evaluation.

Wikoff WR, Gangoiti JA, Barshop BA, et al. Metabolomics identifies perturbations in human disorders of propionate metabolism. Clin Chem. 2007;53:2169–2176.

Correspondence: Bruce A. Barshop at bbarshop@ucsd.edu and Gary Siuzdak at siuzdak@scripps.edu