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Maternal iodine therapy appears safe for thyroid function in breast-feeding offspring
Inorganic iodine therapy administered to mothers with Graves’ disease did not affect thyroid function in most infants, despite high levels of iodine exposure measured via breast milk, according to a study conducted in Japan.
“In daily clinical practice, inorganic iodine therapy may be administered to pregnant and lactating women with Graves’ disease for whom adverse reactions preclude the use of antithyroid drugs,” Katsuhiko Hamada, MD, PhD, of the Tajiri Thyroid Clinic in Kumamoto, Japan, and colleagues wrote. “However, this inorganic iodine is transferred to fetuses via the placenta and infants via breast milk. Therefore, it is important to know the effects of this type of therapy on fetal and infant thyroid function.”
Hamada and colleagues analyzed data from 26 infants of 23 lactating mothers with Graves’ disease treated with potassium iodide for postpartum thyrotoxicosis between September 2012 and August 2015 (median age, 3 months; 14 boys; median potassium iodide dose, 50 mg per day). Researchers collected blood samples from infants using a filter paper procedure, measuring free thyroxine and thyroid-stimulating hormone, as well as breast milk and infant urine iodine concentrations measured on the same day. Subclinical hypothyroidism was defined as TSH level at least 10 µIU/mL in infants aged 6 months or younger, or at least 5 µIU/mL in infants aged 6 to 12 months.
Within the cohort, 19 infants were exclusively breast-fed, and seven infants received mixed feedings; TSH levels and iodine concentrations did not differ between groups.
Researchers detected high median iodine concentrations in breast milk (median, 15,050 µg/L) and in infant urine (median, 15,650 µg/L), and potassium iodide doses correlated with iodine concentrations in breast milk and urine (P < .001 for both). All but one infant had normal thyroid function; median infant TSH level was 2.1 µIU/mL. One infant, aged 5 months, presented with subclinical hypothyroidism that resolved 2 months after maternal potassium iodide discontinuation.
“We note the scarcity of available data regarding inorganic iodine therapy for lactating mothers with Graves’ disease, and suggest that therapeutic indications should be considered carefully,” the researchers wrote. “However, inorganic iodine therapy appears to be a potentially useful alternative for mothers with Graves’ disease with adverse reactions to antithyroid drugs who are contraindicated for iodine-131 therapy because they wish to continue breast-feeding and do not wish to undergo surgery.”
The researchers noted that the Japanese diet includes high daily levels of iodine ingested via seaweed, and that infants living in regions characterized by low iodine intakes and iodine deficiency might be more strongly affected by iodine overload. – by Regina Schaffer
Disclosures: The authors report no relevant financial disclosures.
Perspective
Back to TopThe study by Hamada and colleagues investigated effects of potassium iodide treatment in pregnant and postpartum women in Japan on thyroid function tests in infants. Despite elevated iodine levels in breast milk and infant urine, thyroid studies were normal in most of the infants. Iodine readily crosses the placenta and, in physiological concentrations, is essential for fetal thyroid function and brain development. However, due to a phenomenon known as the Wolff-Chaikoff effect, excess intrathyroidal iodine levels can inhibit thyroid peroxidase mRNA and protein, leading to inhibition of organification and a transient decrease in thyroid hormone production to protect against overproduction due to iodine excess. In adults and older children, an escape from the acute Wolff-Chaikoff effect usually occurs after a few days of exposure to excess iodine to protect against the development of hypothyroidism. This adaptive response leads to decreased intrathyroidal iodine content and a resumption of normal thyroid hormone production. The immature neonatal thyroid gland, on the other hand, is unable to escape from the acute Wolff-Chaikoff effect. This delayed or absent escape from the acute Wolff-Chaikoff effect makes the infant and fetus more susceptible to iodine-induced hypothyroidism. The authors report that newborn screening tests for congenital hypothyroidism were all normal, as reported by the infants’ mothers, even those who had been treated with potassium iodide during pregnancy. This finding is surprising; however, the authors did not include the actual values of the newborn screening thyroid tests and data pertaining to the timing of the potassium iodide dosing in relation to date of delivery. This information would have been more informative as to whether the high maternal iodine ingestion had any effect on fetal thyroid function. Most infants included in the study had thyroid function tests reported from a single point in time. Though the infants did have elevated urine iodine levels at the time of the measured thyroid function tests (presumably transmitted through nursing), the authors did not include data pertaining to the length of time the infants’ mothers had been treated with potassium iodide prior to these measurements. In addition, the majority of the infants were aged at least 2 months. Thus, the normal infant TSH levels may be a reflection of the infants having escaped the Wolff-Chaikoff effect. Measurement of thyroid function tests in the days after the mothers started treatment with potassium iodide and in younger infants would add strength to this report. The authors conclude that Japan is in general an iodine-sufficient region, and as such, this population may be at lower risk for iodine-induced hypothyroidism compared with adults and children in more iodine-deficient areas. This study certainly suggests that the fetus or neonate can, in fact, escape from the Wolff-Chaikoff effect, counter to previous teaching. Future studies measuring thyroid function tests and breast milk and urine iodine soon after birth and in the days after initiating potassium iodide treatment in breast-feeding mothers would add to the author’s conclusions.