To      :  Dawn Staudt-Vanek
Re      :  Thyroid Scan

A thyroid scan is done to delineate structural abnormalities of the thyroid
gland.  As such, two radioactive substances can be used,  Technetium 99M and
Iodine-131.  Technetium has a short radioactive half-life of only 6.7 hours and
if used in a breastfeeding mother,  she could probably safely begin lactating
after 5 half-lives (33 hours).

On the other hand, if I-131 is used breastfeeding is more problematic.  The
amount of I-131 used for a thryoid scan is much much less than is used to
ablate or destroy a thryoid (in hyperthyroidism, Graves disease).  However,
iodides are largely sequestered by the thyroid gland, breast tissues, and milk.
Iodine milk:plasma ratios are extremely high 4-25 depending on timing and whose
reference you read.  The radioactive half-life is 8.1 days.  The biologic
half-life is less but it depends largely on the individual.  To be safe,  one
should theoretically wait until the iodine-131 levels in breastmilk are down to
background levels.  We don't know exactly how long this will take,  but
probably 3-4 weeks depending on the individual.  The absolute answer to this
problem is simply to count the radioactivity in the milk,  and relactate when
it is near background. To do so requires a gamma counter commonly found in many
hospital laboratories.  If the mother is close to a large city,  you can
probably find someone who has such a gamma counter.

However, in those women who are ungoing thyroid ablation with high doses of
I-131 and are breastfeeding,  there is some concern that the concentration of
gamma emiting I-131 in breast tissue could be detrimental (cancer),  and those
women may need to stop breastfeeding prior to undergoing treatment to reduce
exposure of the breast tissue to radiation.  But the doses used in scanning are
much less and this may not be required in this situation.

As per the statement that it "is always around"... this is incorrect.  98% of
the radioactivity would be gone after 5 half-lives or approximately (40.5
days).

Hope this helps.  Below are some references.

Best Regards
Tom Hale, Ph.D.


1.      American Academy of Pediatrics, Committee on Drugs. Transfer of drugs
and other chemicals into human milk.  Pediatrics 93(1):137-150, 1994.
2.      Palmer, KE.  Excretion of 125-I in breast milk following administration
of labeled fibrinogen.  Br. J. Radiol. 52:672, 1979.
3.      Karjalainen, P, et.al. The amount and form of radioactivity in human
milk after lung scanning, renography and placental localization by 131-I
labeled tracers.  Acta Obstet Gynecol Scand 50:357, 1971.
4.      Hedrick WR. et.al. Radiation dosimetry from breast milk excretion of
radioiodine and pertechnetate. J. Nucl. Med. 27:1569, 1986.
5.       Romney B, Nickoloff EL, Esser PD. Excretion of radioiodine in breast
milk.  J. Nucl. Med. 30:124-6,1989.
6.      Robinson PS, Barker P, Campbell A, et.al. Iodine-131 in breast milk
following therapy for thyroid carcinoma. J. Nuci. Med. 35:1797-1801, 1994.