Summary of Use during Lactation

Information in this record refers to the use of sodium iodide I 131 as a diagnostic and therapeutic agent. The American Thyroid Association recommends using I 123 or Tc99m pertechnetate scans for diagnosis of hyperthyroidism in nursing mothers, with Tc99m preferred because of its shorter half-life.[1] The use of I 131 is generally considered to be contraindicated during lactation and breastfeeding should be discontinued permanently for this child after administration of sodium iodide I 131 in a dose of 0.01 MBq (0.004 mCi) or greater to a nursing mother.[1-6] Ceasing breastfeeding at least 6 to 12 weeks before receiving a therapeutic dose of sodium iodide I 131 is recommended to reduce the radiation dose to the breasts and the risk of milk leakage that can contaminate clothing with radioactive iodine.[1-6] Breastfeeding after receiving I 131 would decrease the dose to the target organ (e.g., thyroid) and could decrease the effectiveness of the treatment. Nursing mothers should not work with substances containing I 131 in their workplace.[7]

Parents should refrain from close contact with their infants after therapeutic iodine I 131 administration. The exact duration depends on the dose administered, condition being treated, and source of the recommendation.[8,9] Recommended times range from 15 to 27 days after hyperthyroidism treatment, 16 to 24 days after ablation of thyroid cancer, and 4 to 5 days after follow-up therapy of thyroid ablation therapy.[9]

Drug Levels

I 131 is a beta and high-energy gamma emitter with a main gamma emission energy of 364 keV and a physical half-life of 8.04 days.[10] Estimates of the average effective half-life of sodium iodide I 131 vary from 9.2 hours to 14 hours, and it may be as long as 48 hours in some patients. Iodide is actively secreted into breastmilk by the NaI symporter, which is stimulated by lactation, and actively taken up by the mother's and infant's thyroid glands.[11] About 31% (range 25% to 48%) of administered radioactivity is excreted into breastmilk after administration of sodium iodide I 131.[12,13]

One 29-year-old woman was given 3574 MBq of I 131 to treat thyroid cancer. The radioactivity was measured in all breastmilk expressed over 22 days (105 samples) and one additional sample at day 44. Whole-body and SPECT-CT images were acquired 44 hours after administration and the breast dosage was calculated. Infant exposure was estimated as the sum of external radiation (contact) and internal contamination (ingestion), using the 1 mSv total effective dose limit for the public as a safety threshold. Breastmilk I 131 was excreted with biphasic clearance with half-lives 0.49 and 1.82 days. The excretion into milk represents approximately 15% of the administered activity, which could also decrease the effectiveness of the drug in the mother by 15%. Breast absorbed dose was 569 mGy. The estimated total infant radiation dosage ranged from 0.62 to 688 mSv and thyroid dose from 0.1 mGy to 13,400 mGy, depending on breastfeeding timing. The total dosage stabilized 20 to 30 days after administration. Modeling showed that resuming breastfeeding 9 to 10 days after therapy keeps infant dosage under 1 mSv and infant thyroid dose below 10 mGy. Using a stricter safety criterion of radioactivity less than twice the background level of 0.23 Bq/mL), resumption was proposed at 51 days.[14]

Effects in Breastfed Infants

Relevant published information was not found as of the revision date.

Effects on Lactation and Breastmilk

Relevant published information was not found as of the revision date.

Alternate Drugs to Consider

(Hyperthyroidism Diagnosis) Sodium Pertechnetate Tc 99m, Sodium Iodide I 123; (Hyperthyroidism Treatment) Propylthiouracil, Methimazole

References

1.

Korevaar TIM, Leung AM, Alexander EK, et al. American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum. Thyroid 2026;36:481–544. [PubMed: 42219800]

2.

Mattsson S, Johansson L, Leide Svegborn S, et al. Radiation dose to patients from radiopharmaceuticals: A compendium of current information related to frequently used substances. ICRP Publication 128. Annex D. Recommendations on breast-feeding interruptions. Ann ICRP 2015;44 (2 Suppl):7–321. [PubMed: 26069086]

3.

International Atomic Energy Agency. Radiation Protection and Safety in Medical Uses of Ionizing Radiation, IAEA Safety Standards Series No. SSG-46, IAEA, Vienna. 2018 https://www​.iaea.org​/publications/11102/radiation-protection-and-safety-in-medical-uses-of-ionizing-radiation

4.

ARSAC notes for guidance: Good clinical practice in nuclear medicine. Notes for guidance on the clinical administration of radiopharmaceuticals and use of sealed radioactive sources. 2020 https://www​.gov.uk/government​/publications​/arsac-notes-for-guidance [PubMed: 10732169]

5.

Mitchell KB, Fleming MM, Anderson PO, et al. ABM Clinical Protocol #30: Radiology and nuclear medicine studies in lactating women. Breastfeed Med 2019;14:290–4. [PubMed: 31107104]

6.

Trout AT, Boike TP, Avram AM, et al. ACR-ACNM-ARS-SNMMI-SPR Practice Parameter for Treatment of Benign and Malignant Thyroid Disease With I-131 Sodium Iodide. Am J Clin Oncol 2026;49:51–61. [PubMed: 40694360]

7.

Almén A, Mattsson S. Radiological protection of foetuses and breast-fed children of occupationally exposed women in nuclear medicine - Challenges for hospitals. Phys Med 2017;43:172–7. [PubMed: 28882410]

8.

Siegel JA, Marcus CS, Stabin MG. Licensee over-reliance on conservatisms in NRC guidance regarding the release of patients treated with 131I. Health Phys 2007;93:667–77. [PubMed: 17993847]

9.

Anon. ICRP Publication 94. Release of patients after therapy with unsealed radionuclides. Ann ICRP 2004;34:v–vi, 1–79. [PubMed: 15571759]

10.

Dilsizian V, Metter D, Palestro C, et al. Advisory Committee on Medical Uses of Isotopes (ACMUI) Sub-Committee on Nursing Mother Guidelines for the Medical Administration of Radioactive Material. Final report submitted: January 31, 2019. 2019https://www​.nrc.gov/docs​/ML1903/ML19038A498.pdf

11.

Ramesh S, Basu S. Differential physiological sodium iodide symporter expression in lactating breasts. J Assoc Physicians India 2022;70:11–2. [PubMed: 36082732]

12.

Mountford PJ, Coakley AJ. A review of the secretion of radioactivity in human breast milk: Data, quantitative analysis and recommendations. Nucl Med Commun 1989;10:15–27. [PubMed: 2645546]

13.

Leide-Svegborn S, Ahlgren L, Johansson L, et al. Excretion of radionuclides in human breast milk after nuclear medicine examinations. Biokinetic and dosimetric data and recommendations on breastfeeding interruption. Eur J Nucl Med Mol Imaging 2016;43:808–21. [PubMed: 26732471]

14.

Chaib S, Castellnou S, Keniza T, et al. Estimation of the delay for resumption of breastfeeding after 131-iodine therapy by milk radioactivity measurement. Eur Thyroid J 2026 [PubMed: 42340811]

Substance Name

Sodium Iodide I 131

CAS Registry Number

7790-26-3

Drug Class

Breast Feeding

Lactation

Milk, Human

Radiopharmaceuticals

Iodine Radioisotopes