Iodine 123

Radioactive iodine is concentrated in thyroid and breast milk and may suppress thyroid function in infants.

The I-123 is used as a diagnostic agent. Its mean physical radioactive lifetime is 13.2 hours (Howe 2008) and the half-life of biodegradation is 4.8 to 10.2 hours (Stabin 2000). After 10 physical half-lives (66 hours), the element ceases to have significant radioactivity and after 5 biological half-lives (51 horas) its concentration in the milk is negligible.

If you want to limit exposure to 0 mili Servers (mSv), you should stop breast-feeding for a period equivalent to from 5 to 10 biological half-lives. It is also possible to determine the radioactivity in milk samples.
Since the annual natural background radiation we receive is around 6 mSv, for a more conservative approach we should wait just long enough to achieve an infant exposure of less than 1 mSv (0.1 rem)

The iodine-123 that is administered to nursing mothers must be pure, that means, without containing traces of I-124 and I-125 (Mitchell 2019).

Recommended time for transient weaning to ensure reducing exposure to 1 mSv:

I-123 as sodium iodine hippurate or orto-iodo hippurate (123 I-OIH): Doses up to 100 MBq (4 mCi) waiting time= 8 to 12 hours. According to Nuclear Regulatory Commission of the United States: it is not necessary to stop breastfeeding.

I-123 as sodium iodide (123 I-NaI). If uncontaminated with I 125: doses up to 20 MBq (0.5 mCi) waiting time= 3 to 6 hours. If contaminated: ICRP (Mattsson 2015, Sachs 2013, ICRP 2008): > 3 weeks, The American Thyroid Association (Alexander 2017 Q91 R77): 3 to 4 days

I-123 as iobenguane (I-123 meta-iodobenzylguanidine, I-123-MIBG). If uncontaminated I 123, discontinue breastfeeding for 12, 24 or 48 hours depending on whether the dose was 150, 370 or 400 MBq (4, 10 or 11 mCi). With contamination of I 124 or I 125, the interruption period is >3 weeks (Sachs 2013, ICRP 2008).

I-123 as ioflupane: Stop breastfeeding from 1 day (Society of Nuclear Medicine, Djang 2012) to 6 days (the manufactured).

I 123-Labeled Human Serum Albumin (123 I-HSA): > 3 weeks (Sachs 2013, ICRP 2008).

I 123-Iodofiltic Acid (beta-methyl-15-(4-iodophenyl) pentadecanoic acid; I 123 BMIPP): > 3 weeks (Sachs 2013, ICRP 2008).

I 123-phenylpentadecanoic acid (123 I-IPPA): > 3 weeks (Sachs 2013, ICRP 2008).

For more safety, you can request to measure the amount of radioactivity of the milk in the nuclear medicine service.

If you want to continue breastfeeding, extraction and storage of breast milk in a freezer for days or weeks must be previously ensured. During the suggested days for interruption of breastfeeding after the test is done, the child can be fed exclusively with the milk previously stored or using both breast milk and formula.
After the test you can continue extracting and storing the milk in a separated freezer for 10 physical half-lives (132 hours = 5.5 days, once radiation is gone and it can be used for feeding the baby).

Mothers receiving a dose as lower as 400 MBq for a thyroid scan are not in need to refrain from close contact with their children.