Dr. Hale's book says:
I-125 present more than 12 days
I-131 present 2-14 days
I-123 & its product I-124 present up to 7 days
Half-life of I-131 is 8.1 days
Half-life of I-125 is 60.2 days
Biological half-life may be shorter due to excretion in urine, feces, and
milk.  Suggest to mothers that they pump & store milk prior to exposure.
Another study of I-131 use in a mother was followed for 32 days.  These
authors recommend discontinuing breastfeeding for up to 50-52 days, to ensure
safety of the infant thyroid.  When radionuclide evaluation of the thyroid is
essential, technetium-99m pertechnetate is the preferred agent.  AAP Reason
for Concern.
Kathe Catone, IBCLC, LLLL