More than adequate and excessive iodine

Iodine-induced thyrotoxicosis in apparently normal thyroid glands has been reported in two patients after exposure to 2—10 mg iodine daily for 2—12 months before thyrotoxicosis was diagnosed. Although the pathogenesis of this kind of IIH remains obscure, the positive family history for thyroid disease in both patients may indicate a latent abnormality of their thyroid glands (Skare and Frey, 1980). Moreover, 10 further cases of iodine-induced thyrotoxicosis in apparently normal thyroid glands have been reported (Savoie et al, 1975). In contrast, a more-than-adequate or excessive iodine intake may lead to hypothyroidism and autoimmune thyroiditis, as shown in China in areas with more than adequate and excessive iodine consumption (Teng et al., 2006). 

More than Adequate and Excessive Iodine Intake would Increase the Incidence of Hypothyroidism... 

Teng et al. (2006) explored the effect of iodine intake on thyroid diseases in China. Baseline characteristics of three populations were estabfished in three communities in 1999 and then again 5 years later. The communities had different levels of iodine nutrition mild deficiency more than adequate and excessive intake. Salt iodization had been implemented in China in 1996. In the general population, median UI increased from 165 pg/1 in 1995 to approximately 300 pg/1 in 1999. The concern was with oversupplementation of iodine to a level that is more than adequate, in a region in which iodine intake was previously mildly deficient, which in turn may accelerate the development of subclinical hypothyroidism to overt hypothyroidism. High levels of iodine intake may increase the incidence and prevalence of autoimmune thyroiditis, making it imperative to tailor supplementation needs to each region. 

An unfavorable result found in these surveys was that a significant number of the children, schoolchildren and pregnant women had more than adequate or excessive iodine intake. In a population characterized by long-standing iodine deficiency and a rapid increase in iodine intake, there is a risk of iodine-induced hyperthyroidism. 

More than adequate and excessive levels of iodine intake may promote the onset of hypothyroidism in subjects with high levels of thyroid autoantibodies. 

A new recommended daily iodine intake of 250pg/day was arrived at that represents an increase from the previous figure of 200pg/day. In the pregnant woman, a median UI of <150pg/l is regarded as insufficient, an excretion of 150-249 as adequate, that of 250-499 as more than adequate and >500 as excessive. 

In a follow-up study, 3761 randomly selected subjects, above 13 years, living in one of three regions in China with mild ID (median urinary iodine excretion 84p,g/l), more than adequate iodine intake (urinary iodine 243pg/l), and excessive iodine intake (urinary iodine 651 p.g/1) were included (Teng et ai, 2006). At baseline, the prevalence rates of high level of TPO-Ab (TPO-Ab > 50 lU/ml)... 

There were no statistically significant differences P > 0.05) between urinary iodine concentration when data were adjusted for sex and region, except for the group of children with mild iodine deficiency, between boys with more than adequate iodine intake and between girls with excessive iodine intake P < 0.05) (Figure 120.5). 

Figure 125.2 The cumulative incidence of supranormai thyrotropin levels among subjects with euthyroidism and high levels of TPO antibody or thyrogiobuiin antibody iodine intake was mildly deficient in Panshan, more than adequate in Zhangwu, and excessive in Huanghua. The cumulative incidence increased with increasing iodine intake.

Both more than adequate (MUI, 201-300 pg/1) and excessive (MUI > 300 pg/1) levels of iodine intake may increase the incidence of hypothyroidism. 

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