SODIUM IODIDE 131I

Drugs in Pregnancy and Lactation.

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Name: SODIUM IODIDE 131I
Class: Radiopharmaceutical/Antithyroid
Risk Factor:    X

Fetal Risk Summary

Iodine-131–labeled sodium iodide (Na131I) is a radiopharmaceutical agent used for diagnostic procedures and for therapeutic destruction of thyroid tissue. The diagnostic dose is approximately one-thousandth of the therapeutic dose. Like all iodides, the drug concentrates in the thyroid gland. 131I readily crosses the placenta. The fetal thyroid is able to accumulate 131I by about the 12th week of gestation (1,2 and 3). At term, the maternal serum:cord blood ratio is 1 (4).

As suggested by the above studies on uptake of 131I in fetal thyroids, maternal treatment with radioiodine early in the 1st trimester should not pose a significant danger to the fetus. Two reports describing Na131I therapy at 4 and 8 weeks' gestation resulting in normal infants seemingly confirmed the lack of risk (5,6). However, a newborn, who was exposed to Na131I at about 2 weeks' gestation, has been described as having a large head, exophthalmia, and thick, myxedematous-like skin (7). The infant died shortly after birth. In another early report, exposure to a diagnostic dose of Na131I during the middle of the 1st trimester was considered the cause of anomalies observed in the newborn, including microcephaly, hydrocephaly, dysplasia of the hip joints, and club foot (8). Finally, Na131I administered 1–3 days before conception was suggested as the cause of a spontaneous abortion at the end of the 1st trimester (9). All three of these latter reports must be viewed with caution because of the uniqueness of the effects and the timing of the exposure. Factors other than radioiodine may have been involved.

Therapeutic doses of radioiodine administered near the end of the 1st trimester (12 weeks) or beyond usually result in partial or complete abolition of the fetal thyroid gland (10,11,12,13,14,15,16,17,18,19 and 20). This effect is dose dependent, however, as one mother was treated at 19 weeks' gestation with 6.1 mCi of Na131I apparently without causing fetal harm (2). In the pregnancies terminating with a hypothyroid infant, Na131I doses ranged from 10 to 225 mCi (10,11,12,13,14,15,16,17,18,19 and 20). Clinical features observed at or shortly after birth in 10 of the 12 newborns were consistent with congenital hypothyroidism. One of these infants was also discovered to have hypoparathyroidism (20). In one child, exposed in utero to repeated small doses during a 5-week period (total dose 12.2 mCi), hypothyroidism did not become evident until 4 years of age (17). Unusual anomalies observed in another infant included hydrocephaly, cardiopathy, genital hypotrophy, and a limb deformity (15).

In a 1998 case report, a woman received 500 MBq of 131I in the 20th gestational week (21). Based on gamma camera examinations, the fetal thyroid gland uptake at 24 hours was estimated to be 10 MBq (2% of the dose) or an absorbed dose to the gland of 600 Gy (an ablative dose). The absorbed dose to the fetal body and brain was approximately 100 mGy, whereas the fetal gonads received about 40 mGy. At term, a hypothyroid, but otherwise healthy 3150-g male infant was delivered. Treatment with thyroxine was begun at 14 days of age. A neuropsychological examination at 8 years of age revealed normal mental performance, but with a low attention score and subnormal capacity regarding figurative memory. Plans were made for long-term surveillance of the infant because of the potential for thyroid cancer (21).

In summary, Na131I is a proven human teratogen. Because the effects of even small doses are not predictable, the use of the drug for diagnostic and therapeutic purposes should be avoided during pregnancy.

Breast Feeding Summary

Iodine-131–labeled sodium iodide (Na131I) is concentrated in breast milk (22,23,24 and 25). Na125I also appears in milk in significant quantities (26,27). Uptake of 131I contained in milk by an infant's thyroid gland has been observed (22). The time required for elimination of radioiodine from the milk may be as long as 14 days. Since this exposure may result in damage to the nursing infant's thyroid, including an increased risk of thyroid cancer, breast feeding should be stopped until radioactivity is no longer present in the milk (28).

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References

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  2. Hodges RE, Evans TC, Bradbury JT, Keettel WC. The accumulation of radioactive iodine by human fetal thyroids. J Clin Endocrinol Metab 1955;15:661–7.
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  7. Valensi G, Nahum A. Action de l'iode radio-actif sur le foetus humain. Tunisie med 1958;36:69. As cited in Nishimura H, Tanimura T. Clinical Aspects of the Teratogenicity of Drugs. New York, NY: American Elsevier, 1976:260.
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  9. Berger M, Briere J. Les dangers de la therapeutique par l'iode radioactif au debut d'une grossesse ignoree. Bull Med leg Toxicol Med 1967;10:37. As cited in Nishimura H, Tanimura T. Clinical Aspects of the Teratogenicity of Drugs. New York, NY: American Elsevier, 1976:260.
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  14. Pfannenstiel P, Andrews GA, Brown DW. Congenital hypothyroidism from intrauterine 131I damage. In Cassalino C, Andreoli M, eds. Current Topics in Thyroid Research. New York, NY: Academic Press, 1965:749. As cited in Nishimura H, Tanimura T. Clinical Aspects of the Teratogenicity of Drugs. New York: American Elsevier, 1976:260.
  15. Sirbu P, Macarie E, Isaia V, Zugravesco A. L'influence de l'iode radio-actif sur le foetus. Bull Fed Soc Gynecol Obstet Franc 1968;20: Suppl 314. As cited in Nishimura H, Tanimura T. Clinical Aspects of the Teratogenicity of Drugs. New York, NY: American Elsevier, 1976:260.
  16. Hollingsworth DR, Austin E. Thyroxine derivatives in amniotic fluid. J Pediatr 1971;79:923–9.
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  19. Exss R, Graewe B. Congenital athyroidism in the newborn infant from intra-uterine radioiodine action. Biol Neonate 1974;24:289–91.
  20. Richards GE, Brewer ED, Conley SB, Saldana LR. Combined hypothyroidism and hypoparathyroidism in an infant after maternal 131I administration. J Pediatr 1981;99:141–43.
  21. Berg GEB, Nystrom EH, Jacobsson L, Lindberg S, Lindstedt RG, Mattsson S, Niklasson CA, Noren AH, Westphal OGA. Radioiodine treatment of hyperthyroidism in a pregnant woman. J Nucl Med 1998;39:357–61.
  22. Nurnberger CE, Lipscomb A. Transmission of radioiodine (I131) to infants through human maternal milk. JAMA 1952;150:1398–1400.
  23. Miller H, Weetch RS. The excretion of radioactive iodine in human milk. Lancet 1955;2:1013.
  24. Weaver JC, Kamm ML, Dobson RL. Excretion of radioiodine in human milk. JAMA 1960;173:872–5.
  25. Karjalainen P, Penttila IM, Pystynen P. The amount and form of radioactivity in human milk after lung scanning, renography and placental localization by 131I labelled tracers. Acta Obstet Gynecol Scand 1971;50:357–61.
  26. Bland EP, Crawford JS, Docker MF, Farr RF. Radioactive iodine uptake by thyroid of breast-fed infants after maternal blood-volume measurements. Lancet 1969;2:1039–41.
  27. Palmer KE. Excretion of 125I in breast milk following administration of labelled fibrinogen. Br J Radiol 1979;52:672–3.
  28. Committee on Drugs, American Academy of Pediatrics. The transfer of drugs and other chemicals into human milk. Pediatrics 1994;93:137–50.

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