Studies of Hypothyroidism in Patients with High Iodine Intake

Abstract

Twenty-two patients with spontaneously occurring primary hypothyroidism were studied to evaluate the spontaneous reversibility of the hypothyroid state. Twelve (54.5%) became euthyroid after restriction of iodine intake for 3 weeks (reversible type). In the remaining 10 patients, thyroid function did not improve with restriction of iodine alone, and thus, replacement therapy was required, (irreversible type). In the reversible type, 1) radioactive iodine uptake after 1 week of restricted iodine intake was higher than in the irreversible type [50.0 +/- 12.2% (+/- SD) vs. 4.3 +/- 3.2%; P less than 0.01], 2) the perchlorate discharge test was positive in 2 of 10 patients, and 3) the iodine-perchlorate discharge test, carried out in 7 of 8 patients with negative perchlorate discharge test, was positive in 6. Seven patients with the reversible type were given 25 mg iodine daily for 2-4 weeks; all became hypothyroid again. Two patients had a history of habitual ingestion of seaweed (25.4 and 43.1 mg iodine, respectively), but the remaining 10 patients ingested ordinary amounts of iodine (1-5 mg) daily. The patients with reversible hypothyroidism had focal lymphocytic thyroiditis changes in the thyroid biopsy specimen, whereas those with irreversible hypothyroidism had more severe destruction of the thyroid gland. These results indicate the existence of a reversible type of hypothyroidism sensitive to iodine restriction and characterized by relatively minor changes in lymphocytic thyroiditis histologically. Attention should be directed to this type of hypothyroidism, because thyroid function may revert to normal with iodine restriction alone.

Linking iodine with autoimmune thyroiditis.

“…Conversely, in at least one clinical study, restriction of dietary iodine reversed hypothyroidism in 12 of 22 patients. When seven of the patients with reversed hypothyroidism were re-fed iodine, all of them became hypothyroid again (21).…”

Thyroid Disruptors: How They Act and How We React

“…Iodide-induced goiter occurred in approximately 10% in the coastal regions of Hokkaido, a northern island of Japan (Higuchi 1964;Suzuki et al 1965). Goiter has now disappeared after the restriction of intake of kelp products in the affected areas (Tajiri et al 1986;Yoshinari et al 1983). However, Kono et al reported that the prevalence of nonautoimmune hypothyroidism was still 9.7% in coastal areas of the Hokkaido Islands, and it was proportional to iodide intake, whereas the incidence of nonautoimmune thyrotoxicosis was not related to iodide consumption (Konno et al 1994).…”

Use of iodine for water disinfection: iodine toxicity and maximum recommended dose.

“…Hypothyroidism is attributed to the prolonged suppression of thyroid hormone production as the result of excess iodine levels, but the mechanism through which iodide goiter is produced is not well understood (36,37). Apparently (38,39). The problem may result from the thyroid gland's inability to limit the uptake of iodine when large amounts are available, after which the accumulated iodine inhibits the synthesis of hormone (36) Excess iodine ingestion may be causally related to autoimmune thyroid disease (1,5).…”

“…Various investigators have reported an incidence of hypothyroidism of 2-10% among American patients taking amiodarone (33). The pathophysiologic mechanism by which amiodarone leads to iodine-induced goiter may include the inducement of antithyroid antibodies and direct effects of the medication rather than simple suppression by iodine (32,39,46 Human experimental studies provide the best dose-response data, but most are of limited duration (Table 2). In several recent studies, researchers administered approximately 30 mg iodine/day for 1-12 weeks to healthy volunteers.…”

“…Clinical hypothyroidism is rare among Japanese with endemic goiter due to long-term ingestion of excess iodine. In these cases, both thyroid function and thyroid size return to normal when excess iodine consumption is stopped, unless underlying thyroid disease is present (36,38,39,43).…”