Iodine overload

Castaing H, Fournet JP, Leger FA, Kiesgen F, Piette C, Dupard MC, Savoie JC. Thyroide du nouveau ne et surcharge en iode apres la naissance. [The thyroid gland of the newborn infant and postnatal iodine overload.] Arch Fr Pediatr 1979 36(4) 356-68. 

Species differ widely in their susceptibility to iodine toxicity, but all species can tolerate iodine levels far in excess of their normative requirement. The main problem of iodine intoxication in farm animals is not poisoning but the iodine overload of humans through milk, dairy products and eggs (Anke et al. 1994, Burkhard et al. 2002). 

Determination of the thyroid iodine content by X-ray fluorescence (XRF) investigation offers a unique possibility to study the intrathyroidal iodine pool. Apart from clinical applications in subjects with thyroid disease, it is of utmost interest to apply the method in situations of iodine deficiency or iodine overload. This review gives special attention to the XRF technique, and also describes the application of XRF in vivo and in vitro. 

The areas of special interest are monitoring of iodination programs, identification of individuals with a small iodine pool who are at special risk of developing hypothyroidism in iodine deficiency, and also investigations of mechanisms of thyroid disease in iodine overload. 

There are several causes for exposure to an iodine excess, caused by iodine-containing antiseptics (10000 pg of iodine/ ml) and radiologic contrast media (250-370 mg of iodine/ ml).The aim is to bring to attention the evidence that even a minor iodine overload may impair thyroid function during a period of development when thyroid hormones are very important for the brain (Ares et aL, 1995, 2007). The precocious diagnosis and treatment of changes in thyroid function associated with iodine excess could have beneficial effects in the prevention of developmental abnormalities. We surest a protocol to evaluate thyroid function when iodine... 

Various effects of excessive iodine intake on thyroid function have been described, but the exact mechanism by which iodine overload induces thyroid dysfunction remains unclear. 

Wolfi ChaikofF effect. Raben (1949) showed that this effect was dependent on the amount of iodine in the thyroid gland and not on the plasma concentration of iodine. The acute inhibitory effect of iodine overload is usually transient. Intrathyroidal iodine decreases within a few days despite a high plasma iodine concentration and thyroid hormone synthesis then returns to the previous level. This resumption of the organification of iodine is known as the escape phenomenon (Wolff, 1969), and it means that hypothyroidism will not develop in most individuals despite excessive iodine intake. 

The permeability of newborn s skin permits iodine overloading when iodine-containing antiseptics are applied... 

There are many reasons why fetuses and newborn infants are more hkely to develop hypothyroidism due to iodine-containing pharmaceuticals. The abihty of the thyroid gland to escape from acute inhibition is defective in the fetus and neonate (Theodoropoulos et al, 1979), so both fetuses and newborns are sensitive to iodine overload. Also, the capacity to synthesize and release thyroid hormones is low in preterm babies (Ares et al, 1997). 

Iodine readily crosses the placenta (Wolff, 1969), and significant amounts of iodine are concentrated by the mammary gland and secreted into the breast milk (Spencer et aly 1986 Koga et aly 1995 Roti and degli Uberti, 2001). The high permeability of the skin of neonates can lead to iodine overload when iodine-containing antiseptics are applied (Mitchell et al, 1991 Pyati et al, 1977). Additionally, the renal clearance of iodine is poor in newborn infants (Aliefendioglu et al, 2006). 

Iodine is readily absorbed when PVP-I is applied to the skin of a newborn infant, because of high cutaneous permeability, and neonates are very sensitive to iodine overload, as described previously. Topical PVP-I therapy is associated with a significant risk of hypothyroidism in neonates, especially very-low-weight babies (Smerdely et ai, 1989). Many cases of hypothyroidism induced by topical use of PVP-I have been reported in newborn infants, mainly from iodine-deficient regions (Markou et ai, 2001). However, a case of severe hypothyroidism in a neonate was also reported from North America, an iodine-sufficient region (Khashu et al. 2005). A premature infant developed severe hypothyroidism that required L-thyroxine replacement therapy after application of PVP-I for 20 days. 

The inorganic form of iodine represents about 0.5 % of the total plasma iodine. The rest occurs in bound form with specific plasma protein (protein - boimd iodine, FBI) which has gained wide use as an indicator of thyroid activity in humans. It has been reported that the total plasma iodine concentration in healthy subjects is between 40 and 80 pg/l. According to Allain s studies when plasma iodine concentrations are below 40 pg/l, hypothyroidism is highly likely, when they are between 80 and 250 pg/1, hyperthyroidism, particularly Graves disease is probable. Above 250 pg/1 - iodine overload is almost certainly indicated (Allain et al., 1993). 

Endocrine Prenatal, perinatal, or postnatal iodine overload can cause problems such as transient hyperthyrotropinemia, hypothyroidism, or hyperthyroidism in preterm and term infants [SED-15, 1896 SEDA-30, 279 SEDA-31, 411 SEDA-32, 440], Severe hypothyroidism in two term neonates resulted from iodine overload caused by postnatal use of an iodine-containing antiseptic [26 ]. One case was caused by excessive application of the antiseptic to the umbilicus and the other from maternal breast milk following the use of an iodine antiseptic by the mother for her episiotomy incision. 

Kurloglu S, Akin L, Akin MA, Coben D. Iodine overload and severe hypothyroidism in two neonates. J Qin Res Pediatr Endocrinol 2009 1(6) 275-7. 

The aim of this paper is to report on thyroid iodine content during subacute thyroiditis and hyperthyroidism due to iodine overload. X-ray fluorescence appears 1 / to be a differential diagnostic tool between this two diseases in which radioiodine uptake is undetectable 2°/ an useful tool in the follow-up of the patients during the course of both diseases. 

Chanoine, J.P., Boulvan, M., Bourdoux, P. et al.. Increase recall rate at screening for congenital hypothyroidism in breast fed infants bom to iodine overloaded mothers. Arch. Dis. Child. 63 1027 (1988). 

We specifically asked the doctors in each medical school to select patients who had not received any kind of medicine containing iodine or medical exams which could lead to iodine overload. It should be noted however that the patients were free to have side dishes in addition to the regular hospital diet. Therefore, such wide variations in urinary excretion may be related to iodine supplements coming from the diet. 

P.C. Clemens and R.S.J. Neumann, Influence of iodine overloading on neonatal thyroid screening results, J Endocrinol Invest 12 841(1989). 

Screening for congenital hypothyroidism frequently shows transient abnormality of thyroid function. The newborn is very susceptible to iodide excess and in 70 % of the cases (1), the reason for transient hypothyroidism is iodine overload (2-3). A common cause may be X ray investigations with iodine products. Two newborns hospitalized in our Neonatal Intensive Unit presented with iodine overload from investigations, producing transient hypothyroidism. The hypothyroidism disappears with iodine elimination. In one case, the child were submitted at six months to another iodine overload without consequences. The thyroid tests show no change. Transient neonatal hyperthyrotropinemia may be due to iodine overload, and the search for its etiology is essential, because it allows one to stop the treatment. 

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