Why do the nodules of a multinodular goiter not show up on a i-123 thyroid uptake scan?

They are "cold": In this case, the nodules are non-functioning, so they do not take up the nuclear tracer.

Scan shows activity: A thyroid scan looks for functioning thyroid tissue. Much of a multinodular goiter can be nonfunctional and thus not show up well or will blend into the background thyroid tissue.

Size and intensity: There are few reasons why you may not see known nodules in a i-123 thyroid scan. First, size is important as usually cameras cannot resolve small nodules that are less than 10-to-7mm. Also, thyroid nodules may be isointense and indistinguishable from the rest of the gland. Cold nodules, if small, are also more difficult to resolve than hot nodules that may stand out more easily.

MNG: "multinodular goiter" refers to an enlarged and nodular thyroid gland. Subcentimeter nodules are usually below the resolution of the thyroid scan. Nodules that are normally functioning may blend into the surrounding normal thyroid tissue and thus are not seen. Usually thyroid scan is performed to evaluate for toxic mng (aka plummer's disease) where there are multiple hyperfunctioning nodules.