Urinary iodine concentration during pregnancy in an area of unstable dietary iodine intake in Switzerland

Abstract

We prospectively investigated urinary iodine concentration (UIC) in pregnant women and in female, non-pregnant controls in the canton of Berne, Switzerland, in 1992. Mean UIC of pregnant women [205 +/- 151 microg iodine/g creatinine (microg l/g Cr); no. = 153] steadily decreased from the first (236 +/- 180 microg l/g Cr; no. = 31) to the third trimester (183 +/- 111 microg l/g Cr, p < 0.0001; no. = 66) and differed significantly from that of the control group (91 +/- 37 microg l/g Cr, p < 0.0001; no. = 119). UIC increased 2.6-fold from levels indicating mild iodine deficiency in controls to the first trimester, demonstrating that high UIC during early gestation does not necessarily reflect a sufficient iodine supply to the overall population. Pregnancy is accompanied by important alterations in the regulation of thyroid function and iodine metabolism. Increased renal iodine clearance during pregnancy may explain increased UIC during early gestation, whereas increased thyroidal iodine clearance as well as the iodine shift from the maternal circulation to the growing fetal-placental unit, which both tend to lower the circulating serum levels of inorganic iodide, probably are the causes of the continuous decrease of UIC over the course of pregnancy. Mean UIC in our control group, as well as in one parallel and several consecutive investigations in the same region in the 1990s, was found to be below the actually recommended threshold, indicating a new tendency towards mild to moderate iodine deficiency. As salt is the main source of dietary iodine in Switzerland, its iodine concentration was therefore increased nationwide in 1998 for the fourth time, following increases in 1922, 1965 and 1980.

Iodine intake in Portuguese pregnant women: results of a countrywide study

“…Similar to our results, one study performed in northern Portugal (31) and another from Spain (34) did not find statistical differences in UIC values between early and late pregnancy. However, other authors have referred a clear decrease (43,44), while others have shown even an increase (45). These discrepancies are probably due to the level of iodine intake and other unclear ethnic nutritional differences in the analyzed women (44).…”

“…However, other authors have referred a clear decrease (43,44), while others have shown even an increase (45). These discrepancies are probably due to the level of iodine intake and other unclear ethnic nutritional differences in the analyzed women (44).…”

Pilot study of urinary iodine concentration and of biochemical thyroid parameters before and after cautious public health intervention on salt iodide content: The Swiss longitudinal 1996–2000 iodine study

“…Thereafter, IC in salt was progressively increased in 1965 (7.5 ppm iodide) and in 1980 (15 ppm iodide). In order to further reduce incidence of IDD such as small goiters occurring during pregnancy and transient neonatal hypothyroidism associated with borderline ID in women in the reproductive age (Als et al, 2000a;Brander et al, 2003), IC in salt was again increased from 15 to 20 ppm (or from 20 to 25 mg potassium iodide/kg salt) by ministerial decree in 1998 in Switzerland. The reason why the 1998 increase of IC in salt was rather small was that negative secondary effects were to be avoided and thereby political acceptance increased (Ermans et al, 1995;Bourdoux et al, 1996;WHO, 1997;Stanbury et al, 1998).…”

“…Urinary iodine concentration (UIC) represents a good approximation of iodine supply; median UIC ought to be 4100 mg/l (Delange, 1998). Former examples of hidden sources of iodine excess, of significant and unexpected modifications of UIC in a population after an intervention on IC in salt or after a change in the consumption of iodized vs noniodized salt point to the importance of performing prospective studies (Ermans et al, 1995;WHO, 1997;Delange, 1998;Zhao et al, 1998;Bürgi et al, 1999;Als et al, 2000a, b;Black, 2003;Brander et al, 2003). Thus, UIC in the NHANES III survey in the US showed an unexpectedly important decrease of UIC between the 1971-1974 and 1988-1994 periods, probably due to a decreased use of iodate as bread stabilizer (Hollowell et al, 1998).…”

11 Hypothyroidism in Pregnancy

“…In these women, total body iodine levels remain stable throughout pregnancy [46]. However, in areas of even mild to moderate iodine deficiency, total body iodine stores, as reflected by urinary iodine values, decline gradually from the first to the third trimester of pregnancy [47].…”