Effect of iodine intake on thyroid diseases in China
It has been 14 years since universal salt iodization was conducted legislatively throughout China in 1996, and the residents experienced the period of excessive iodine intake (MUI>300 μg/l) for 6 years and more than adequate iodine intake (MUI 240 μg/l) for 8 years, and now the status of iodine is normally adequate (MUI 180 μg/l).
During this period, we completed several epidemiological studies on the relationship between iodine intake and thyroid diseases. (1) A 5-year follow-up study (n=3761) conducted in three regions with different levels of iodine intake (MUI 84, 243 and 651 μg/l respectively) showed that the incidence of subclinical hypothyroidism increased by 11.3 and 12.6 times in the communities with more than adequate iodine intake and excessive intake, respectively. The incidence of autoimmune thyroiditis increased by 4.4 and 5.5 times respectively. A shift in iodine intake from mildly deficient to more than adequate was a risk factor for the development of subclinical hypothyroidism to overt hypothyroidism. the incidence of papillary thyroid carcinoma significantly increased in the community with excessive intake of iodine (MUI 651 μg/l). (2) A cross-sectional study (n=3813) conducted in two communities with different levels of iodine intake (MUI 261 and 145 μg/l, respectively) showed that the prevalence of subclinical hypothyroidism and autoimmune thyroiditis were significantly higher in subjects from the region with more than adequate iodine intake than with adequate iodine intake. (3) A follow-up study conducted in pregnant women (n=610) showed that the prevalence of post-partum thyroiditis significantly increased in the subjects with more than adequate and excessive iodine intake. (4) A cross-sectional epidemiologic study of ten cities (n=15 008) showed that the prevalence of overt hypothyroidism, subclinical hypothyroidism, positive TPOAb and TgAb in the region with more than adequate iodine intake (MUI 240 μg/l) were significantly higher than adequate iodine intake (MUI 189 μg/l).The serum TSH level significantly increased with the increase of iodine intake.
In conclusions, supplementation of iodine should be maintained at a safe range with MUI 100200 μg/l in order to ensure the thyroid health of susceptible populations (more than 10% of the general population).It is necessary to establish the region iodine-specific reference interval of TSH to avoid the influence of iodine status.
Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.
Funding: This work was supported, however funding details are unavailable.