Effects of antithyroid drugs on radioiodine treatment: systematic review and meta-analysis of randomised controlled trials

There is a disagreement about the influence of antithyroid drugs on
radioiodine treatment.
In the systematic review and meta-analysis of randomised controlled trials
it was concluded that antithyroid drugs increase rates of failure and
reduce rate of hypothyroidism if they are given in the week before or
after radioiodine treatment (Walter et al 2007). I believe that one
explanation for failure is the fact that antithyroid drugs simply affects
the uptake and residence time of radioiodine. There is a risk in these
cases that the thyroid receives a smaller absorbed dose from radioiodine
if dose calculation including estimation of effective half-life of the
radioiodine is not performed. If a dose calculation is performed, the
outcome will be the same in patients treated with antithyroid drugs as in
those without pre-treatment.

In Sweden the authorities recommend to use individual dose
calculations for radioiodine treatment of patients with hyperthyroidism.
We have thus for many years used a protocol where a test dose is given in
order to calculate the amount of radioactivity (MBq) needed to obtain the
absorbed dose (Gy) aimed at. We have shown that when we aim at an absorbed
dose of 120 Gy we only have a failure of 7 % (Berg et al 96 b). The
individual dose calculation takes into account the thyroid volume, the
iodine uptake after 24 hours and the effective half -life of the
radioiodine. We have reported about the special significance of the
effective half-life (residence time of the radioisotope) in these
measurements (Berg 1996a). We thus showed that the effective half-life can
vary between 2 and 8 days where the mean half-life for patients with
Graves’ disease was 5.3 days without pre medication with antithyroid drugs
whereas patients with medication up to 7 days before treatment had a mean
effective half-life of 4.4%. Thus if the half-life is not considered it is
likely that the patients with prior antithyroid drugs will receive a lower
absorbed dose than prescribed. The shorter period without medication the
more impact on the half-life can be expected.

One explanation for the reduced half-life is that antithyroid drugs
reduce the protein binding of iodine to thyroglobulin at the apical part
of the thyrocytes. The radioiodine will thus reside shorter in the gland.

We often see a high uptake and a short half-life in patients with
prior antithyroid drug treatment. It is our experience however that when
an individual dose calculation is performed the treatment failure is in
the same order for patients with and without prior medication. If
individual dose calculation is not performed and standard activities are
used it is likely that patients with prior antithyroid drugs receive a
lower absorbed dose to the thyroid than wanted. A dose of eg 90 Gy is
simply more unlikely to cure than the aimed dose of 120 Gy.

Berg G, Michanek A, Holmberg E, Nyström E. Clinical outcome of
radioiodine treatment of hyperthyroidism: a follow up study. J Int Med
1996; 239:165-71.

Berg G, Michanek A, Holmberg E, Fink M. Iodine-131 treatment of
hyperthyroidism: significance of effective half-life measurements. J Nucl
Med 1996; 37: 228-32

Competing interests:
None declared