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Administration of pharmacological doses of stable iodine can significantly
decrease thyroid committed dose by the following mechanisms (14):
- by saturating active iodine transport into the thyroid and
diluting the radioiodines taken up by the gland,
- by increasing storage of stable iodine in the thyroid, thus leading
to attainment of a new steady-state at lower level of (radio)iodine uptake,
- by lowering circulating radioiodine and its thyroidal (re)uptake.
It is well known (66) that a single dose of 130 mg of potassium
iodine (KI) completely blocks the thyroid radioiodine uptake approximately
30 minutes after administration and keeps the suppression constant for
about 24-30 hours. Such a dose will be about 25% effective 40 hrs after
administration. It was also shown, that a KI dose of 65 mg is an effective
thyroid blocking dose in children and teenagers (67-71). This dose would
be also effective in adults, although, at least 60 minutes are required
before radioiodine entry into thyroid is blocked (69,70). In addition to
the dose, the time of KI administration is another factor influencing the
effectiveness of thyroid protection. Implementation of KI administration
shortly before, or within 1-2 hours after exposure to radioiodines would
effectively protect the thyroid (68,70). KI given 12 to 24 hrs after the
thyroid exposure to radioiodines will lead to minimal gland protection,
however, limited benefit is still possible. Following previous accidents
in Windscale and Three Miles Island, protective KI administration was not
implemented. After the Chernobyl accident a total of 5.4 million people
including 1.7 million children received KI prophylaxis in regions of former
USSR relatively close to Chernobyl (3,26). The data concerning KI dose
in different age groups, the formula of KI and the period of KI administration,
as well as efficiency of this action was never investigated and published.
It can be only assumed that in Prypyat and a few nearby population centers
iodine prophylaxis was implemented on April 27, 1986 (i.e. one day after
the explosion), prior to the decision of evacuation (26). In Poland, KI
prophylaxis and other protective measures were institutioned on April 29,
as a mandatory action for all children up to 16 years old (72,73). Pregnant
and lactating women were advised to take KI. As was estimated (73) about
10.5 million children and about 7 million adults received a single KI dose.
A KI dose was determined of as 15 mg for newborns, 50 mg for children 5
years or under and 75 mg for all others. A follow-up study revealed (73),
that in those who took KI on April 29, the reduction in thyroid absorbed
radioiodine dose amounted to about 40%, while in those who received KI
on April 30, the thyroid radioiodine dose reduction was about 25%. As already
published in detail (73), the prophylactic measures in Poland were aimed
to reduce the final thyroid committed dose and keep it below the desired
50 mSv limit. The predicted scenario assumed, that the accident was very
serious and would lead to prolonged radioiodine release. The KI administration
was found to be safe. Transient intrathyroidal side effects (mild increase
of serum TSH and decrease of serum fT4) were observed in 12 of 3212 neonates
who were given KI on first day of life. This form of Wolff-Chaikoff phenomenon
completely disappeared after 3-5 days and thyroid function was found normal
2-3 years later (73). The extrathyroidal side-effects were mild and transient.
They were observed in 0.35% of children and 0.2% of adults who received
one or two doses of KI (72,73).
Administration of iodine as prophylaxis to a population requires a number
of planing decisions. The logistics of a policy of implementation of stable
iodide following a nuclear accident have been described in detail elsewhere
(74). One of the most important general actions which is significant to
health, and would also reduce thyroid radioiodine committed dose is appropriate
iodine intake in every day diet. It has been established (75) that in countries
where daily iodine intake exceeds 150 ug, 24 hr 131I thyroid uptake varied
between 19 and 22%. On the contrary in countries with mild or moderate
iodine deficiency 24 hr 131I thyroid uptake lies between 40 and 60% (75).
It must be pointed out that even in countries with appropriate iodine intake
protective measures in case of nuclear accident should be considered and
if necessary implemented at least for high risk groups.
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