Our results indicate that iodine deficiency is present in approximately one third of middle-aged, community-dwelling people living in the city of Lausanne, Switzerland. Factors such as older age, being a woman or presenting with hypertension are associated with iodine deficiency, while higher BMI and smoking are associated with a lower likelihood of iodine deficiency.
Prevalence of iodine deficiency
Median UIC were 131, 123 and 138 μg/L overall, for women and for men, respectively. Our results indicate that the iodine intake of the Swiss population is adequate, as the overall median is above the 100 μg/L threshold. Importantly, the values for women were higher than reported by Andersson et al. among women of reproductive age in 2020 (88 μg/L)18. Indeed, stratifying the analysis according to menopausal status showed a median UIC of 130 and 116 μg/L among non-menopaused and menopaused women, suggesting that the median UIC in Swiss women has decreased.
As previously shown, UIC is considered a reliable tool to estimate iodine status at a population level9,10,11,13, even though its day-to-day variability prevents it from assessing an indivual’s iodine status9. Iodine status represent a challenge for public health as even mild to moderate iodine deficiency can lead to toxic nodular goiter and hyperthyroidism26. Monitoring UIC in order to improve iodine intake is a major component of public measures against thyroid disorders.
In this study, one third to one tenth of participants presented with deficient ioduria, depending on the threshold applied; of those, mostly presented with a mild deficiency. A study conducted on 24-h urine between 2009 and 2013 by Stalder et al.16 using the 100 μg/L threshold reported a prevalence of inadequate iodine intake, defined as an estimated average requirement of 95 μg/day, at 14% for women and 4% for men, values lower than reported in this study.
Altogether, Swiss households seem to be well covered in iodized salt18. Iodized salt represents 44% of the iodine intake of the Swiss population27, and a positive correlation was found between urinary sodium and iodine in this study, suggesting that still a sizable fraction of iodine intake comes from iodized salt. Still, dietary habits tend to evolve toward more processed foods and other sources of salt that are not necessarily iodized28. Iodized salt is more expensive than non-iodized salt, which might explain its decrease in consumption. Indeed, iodized salt contributed only to 24% of total salt intake in the Italian adult population2 and, in this study, participants with iodine deficiency consumed more salt than those with adequate iodine status, suggesting consumption of non-iodized salt. Besides, the decrease of salt consumption in response to prevention campaigns against hypertension represents a new challenge for iodine fortification7. As the consumption of salt and iodine intake are closely related, the reduction of cardiovascular risks and thyroid disorders must be considered jointly2,24. Indeed, a salt intake below 5 g/day as recommended by the WHO will likely fail to achieve adequate iodine intake2. Further, as salt fortification is not compulsory in Switzerland, alternate ways to provide an adequate supply of iodine to the Swiss population must be considered, and some authors suggested the mandatory use of fortified salt in all processed foods2,18.
Factors associated with iodine deficiency
One third of women presented with iodine deficiency as defined by UIC < 100 μg/L, and approximately one in seven (13.0%) by UIC < 60 μg/L. The first finding is in agreement with the literature27,29, and could be explained by lower food intake and lesser use of salt by women relative to men2,30, although this information is lacking in this study.
The risk of iodine deficiency as defined by UIC < 100 μg/L increased with age, a finding in agreement with one study31 but not with another16. One possible explanation are changes in dietary intake associated with recommendations to decrease salt consumption due to increased prevalence of hypertension, as urinary sodium concentrations were lower in older participants.
Participants with hypertension presented more frequently with iodine deficiency, a finding also reported elsewhere32. As participants with hypertension presented a higher mean urinary sodium concentration, the initial hypothesis of restricted salt consumption in this group was challenged. Indeed, a previous study reported that only 8% of people with hypertension report consuming a low-salt diet33, and the Swiss Survey of Salt reported no difference regarding hypertension prevalence between people eating less or above 5 g of salt daily34. Two studies reported that sodium intake and sodium restriction in healthy populations and in participants with hypertension does not lower iodine levels35,36, while another study found a significant increase of iodine deficiency associated with salt restriction in women37. Overall, the associations between urinary sodium and iodine deficiency should be further explored.
Higher BMI was a protective factor against iodine deficiency2,38,39. Again, the most likely explanation is an increased food intake by people with obesity, including an increased consumption of iodine sources, including salt. Indeed, the recent Swiss Survey of Salt reported that people with normal BMI had a lower salt consumption than people with overweight or obesity34.
Surprisingly, smoking was associated with lower risk of deficient UIC. Our findings contradict results from previous studies that reported clear associations between smoking status and iodine intake40,41. This could be due to a higher proportion of older participants and especially older women among non-smokers42, or an increased salt consumption by smokers43. Still, urinary sodium levels were lower among current smokers than nonsmokers, suggesting that other mechanisms or behaviors might occur.
Implication for clinical practice
In Switzerland, the last increase in iodine concentration of salt occurred in 2014 but did not improve iodine coverage in the Swiss population18. Also, given the recommendations to decrease salt intake for cardiovascular prevention, it seems unlikely that a new increase of iodine concentration in salt would allow for a sufficient iodoprophylaxis in the Swiss population. A possible alternative would be to mandate salt iodization of processed foods, although this might require changes in food processing or legislation.
Strengths and limitations
The large sample size constitutes a major strength of this study and can be considered representative of the Lausanne population to a certain extend. UIC is a method of choice to assess iodine status at the population level, as it is less demanding and a reliable alternative to 24 h urine collection. An additional strength is the number of covariates, which allowed for a wide range of analyses and adjustments.
This study also has some limitations. First, it was conducted in a single geographical location, and results might not be generalizable to other settings in Switzerland, as it has been shown that dietary intake varies by linguistic region44. Second, although it is commonly acknowledged that 90% of dietary iodine intake is excreted in urine, this pourcentage could vary with iodine status. Still, we believe that our results remain relevant, particularly for comparison with other studies using UIC. Third, no information regarding thyroid status was collected. Hence, we could not associate the UIC with thyroid disease, and it would be important that future studies perform such analyses. Finally, the data was collected for period 2003–2006, which is relatively old, and changes might have occurred as the dietary intake of the population has improved45. Still, to our knowledge, no recent study assessed the UIC of the Swiss population. More recent studies on salt intake conducted in Switzerland suggest that in both children46 and adults34 have a high intake of salt, but whether this salt contains added iodine was not assessed. It would be important that the iodine status of the Swiss population be evaluated using more recent information.