Iodine induced hypothyroidism

Lithium carbonate, administered for affective and bipolar disorders, may enhance the effects of antithyroid drugs. Potassium iodide, used as an expectorant, is a major ingredient in many cough medications. Iodide derived from this source may enhance the effects of antithyroid drugs and lead to iodine-induced hypothyroidism. Iodine in topical antiseptics and radiological contrast agents may act in a similar manner. 

Iodine-induced hypothyroidism has been described as the result of prolonged intake of potassium iodide and iodinated glycerol as mucolytics (53). 

Although povidone-iodine is no longer used in dialy-sates, a povidone-iodine-containing cap is used to seal the Tenckhoff catheter during the day. Iodine-induced hypothyroidism occurred in a 3-year-old boy and an 18-month-old girl, in both cases due to the sealing cap (10). The povidone-iodine inside the cap diffused into the catheter and flushed into the peritoneal cavity at the next dialysis session. 

Vulsma T, Menzel D, Abbad FC, Gons MH, de Vijlder JJ. Iodine-induced hypothyroidism in infants treated with continuous cyclic peritoneal dialysis. Lancet 1990 336(8718) 812. 

Patients with beta-thalassemia major have an increased risk of primary hypothyroidism. In 23 patients with beta-thalassemia amiodarone was associated with a high risk of overt hypothyroidism (33 versus 3% in controls) (43). This occurred at up to 3 months after starting amiodarone. The risk of subclinical hypothyroidism was similar in the two groups. In one case overt hypothyroidism resolved spontaneously after withdrawal, but the other patients were given thyroxine. After 21-47 months of treatment three patients developed thyrotoxicosis, with remission after withdrawal. There were no cases of hyperthyroidism in the controls. The authors proposed that patients with beta-thalassemia may be more susceptible to iodine-induced hypothyroidism, related to an underlying defect in iodine in the thyroid, perhaps associated with an effect of iron overload. 

Iodine-induced goiter Iodine-induced hypothyroidism Fetus and neonate... 

In addition to iodine measurements, three studies (Abdulla et al., 1981 Key et al., 1992 Raiuna et al., 1994) performed thyroid function tests as indirect measures of iodine sufficiency. Thyroid function was normal in two studies (Abdulla et al., 1981 Rauma et al., 1994). The third study reported elevated thyroid-stimulating hormone (TSH) levels in 5 of 48 vegans, but did not measure dietary or urinary iodine levels (Key et al., 1992). It is noteworthy that the three highest TSH levels were in vegans who usually took kelp it is possible that these three actually suffered iodine-induced hypothyroidism (Wiersinga and Braverman, 2003) secondary to the consumption of excessive iodine-rich kelp. 

Previously published reports have described both sub-clinical and overt thyroid dysfunction as a result of excess iodine ingestion. Moreover, iodine-induced hypothyroidism is not rare, at least in iodine-replete areas. The possible underlying mechanisms of this impairment consist of both the inhibition of thyroid function by excess iodine and the iodine-induced enhancement of thyroid autoimmunity. On the basis that an excess iodine intake may cause hypothyroidism, several study results support iodine restriction for the treatment of hypothyroidism. The success rate of dietary iodine restriction alone, without thyroid hormone replacement, is reported to be approximately 50-80% in patients with primary hypothyroidism due to Hashimotos thyroiditis and other causes. Relatively mild hypothyroidism, high radioactive iodine uptake and uptake, increased free plasma iodine and... 

The possible underlying mechanisms of excess iodine-induced hypothyroidism consist of both the inhibition of thyroid function by excess iodine and the iodine-induced enhancement of thyroid autoimmunity. 

The beneficial effects of iodine supplementation in the prevention and control of developed thyroid abnormalities due to iodine deficiency have been discussed so far in this chapter. However, supplementation with excess iodine, including the improvement of a previous iodine-deficient state, may cause thyroid dysfunctions, viz., iodine-induced hypothyroidism/iodide goiter in susceptible subjects (Roti and Vagenakis, 2000) and iodine-induced hyperthyroidism (IIH) especially in individuals over 40 years of age and who have been iodine deficient for a long period in the past (Vidor et ai, 1973). It may also increase the ratio of papillary/follicular carcinomas (Slowinska-Klencka et ai, 2002). In other words, both low and excess intake of iodine is related to further risk of thyroid disease. Although a daily intake of up to 1000 pg/day by a normal adult individual is quite safe (WHO, 1994), the upper limit is much lower in a population that has been exposed to iodine deficiency in the past. Therefore, to prevent IDD, the recommended iodine requirement in an adult individual is fixed within a narrow range of 150 rg/day (Knudsen et ai, 2000). Iodine supplementation under certain conditions in certain populations causes adverse effects, e.g., iodide goiter and iodine-induced hypothyroidism, IIH, iodine-induced thyroiditis and thyroid cancer. 

Excess iodine supplementation, however, is related to the further risk of thyroid disease. Occurrence of iodine-induced hypothyroidism and iodide-goiter, IIH and iodine-induced thyroiditis are the consequence of excess iodine supplementation in certain populations who were iodine deficient till 40 years or more of age. 

Iodine is an essential component of thyroid hormones, and iodine deficiency can lead to severe hypothyroidism. On the other hand, excessive iodine intake also results in thyroid dysfunction in certain persons. Coindent described the first case of iodine-induced hyperthyroidism in 1821 (Fradkin and Wolff, 1983), and Hurxthal (1945) reported the first case of iodine-induced hypothyroidism. Ingestion of iodine-rich foods, such as seaweed, can cause hyperthyroidism, and the incidence of hyperthyroidism showed an increase in regions of dietary iodine deficiency after prophylactic iodization of bread or salt. Iodine-containing pharmaceuticals, such as povidone-iodine (PVP-I), radiographic contrast media and amiodarone, are a major source of excessive iodine intake (Wolff, 1969 Fradkin and Wolff, 1983 Markou et aL, 2001 Roti and degli Uberti, 2001). 

It has been suggested that iodine-induced hypothyroidism, especially that occurring in patients on amiodar-one therapy, fetuses and neonates, is caused by failure of the thyroid gland to escape from acute inhibition (Pitsiavas etat, 1999 Theodoropoulos 1979). 

Iodine-induced hypothyroidism usually improves within a few weeks after iodine withdrawal, but some patients require temporary hormone replacement therapy (Wemeau, 2002). Unless there is underlying Graves disease, patients with iodine-induced hyperthyroidism do not have exophthalmos (Fradkin and Wolff, 1983 Roti and degU Uberti, 2001). Unlike hypothyroidism, iodine-induced hyperthyroidism is often not self-limited, and medication is required in addition to discontinuation of iodine-containing pharmaceuticals (Rajatanavin et al, 1984). 

The fetus and neonate are both at risk of developing iodine-induced hypothyroidism (Table 96.3). Many cases of such hypothyroidism in fetuses and neonates have been reported, especially in iodine-deficient regions of Europe, but also in iodine-sufficient areas. Iodide goiter in neonates is usually a transient problem. However, tracheal obstruction due to such goiter can be fatal (Markou et al, 2001 Wolff, 1969). Transient hypothyroidism without elevation of thyroid-stimulating hormone (TSH) in extremely... 

Note The fetus and neonate are both at risk of developing iodine-induced hypothyroidism. Many cases of hypothyroidism in newborns have been reported, especially in iodine-deficient regions of Europe, but also in iodine-sufficient areas. 

Iodine-induced hyperthyroidism (especially AIT) is often difficult to treat, whereas iodine-induced hypothyroidism usually resolves after iodide withdrawal. Individuals at risk of developing iodine-induced thyroid dysfunction should be observed carefully if they are administered iodine-containing drugs. 

Table 97.1 Causes of iodine-induced hypothyroidism or goiter...

The same results were found from other studies a study in Poland enrolled 35000 patients who had received fine-needle aspiration biopsy of the thyroid from 1985 to 1999, and statistical analysis indicated that the incidence of AIT rose from 1.5 to 5.7% after USI since 1992 (Slowinska-Klencka et al, 2002). Kahaly et al. (1997) reported that after supplementing iodine 200pg/l to patients with endemic thyroid goiter (MUI, from 30pg/l, rose to 213pg/l), the incidence of iodine-induced hypothyroidism and AIT increased. It was reported from Greece and Sri Lanka that there was an obvious increase in the incidence of AIT in children after iodine supplementation was implemented in areas with deficient iodine intake (Zois et al, 2003 Premawardhana et al, 2000). 

Endocrine The mechanisms of iodine-induced hypothyroidism and sources of excessive iodine exposure including disinfectants were reviewed [72 ]. The use of iodine-containing solutions as hand disinfectants in hospital settings may cause the operating room staff to have significantly higher urinary iodine concentrations [73 ]. 

Table 3. Iodine Induced Hypothyroidism Euthyroid Patients With Underlying Thyroid Disease...

Roti, E., Minelli, R., Gardini, E., Bianconi, L., Neri, T., Gavaruzzi, G., Ugolotti, G., Salvo, D., Braverman, L.E., Impaired intrathyroidal iodine organificaton and iodine-induced hypothyroidism in euthyroid women with a previous episode of postpartum thyroiditis, J. Clin. Endocrinol. Metab. 73 958 (1991). 

Clark, O.H., Cavalieri, R.R., Moser, C., Ingbar, S.H., Iodine-induced hypothyroidism in patients after thyroid resection, Eur. J. Clin. Invest. 20 573 (1990). 

Theodoropoulos, T., Braverman, L.E., Vagenalds, A.G., Iodine-induced hypothyroidism a potential hazard during perinatal life. Science 250 502 (1979). 

I would tend to agree with you. When we estimated the incidence of Hashimoto s thyroiditis in Worcester, a suburb of Boston, we found that about 25 % of women over the age of 60-65 years have thyroid autoantibodies. Therefore, you find already there the potential in a large fraction of the population for iodine-induced hypothyroidism. I would suggest that, at least in North America, excess iodine should probably be avoided in elderly patients. This has been clearly shown in a study on the chronic administration of iodine-rich expectorants in a population of nursing homes. If I remember correctly, the authors reported an incidence of 6-7 % of hypothyroidism induced by iodine excess. 

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