86 Harold L. Atkins hypothyroidism. Consideration of these factors may yield clues to the etiology of post-therapy myxedema and lead to the development of methods to reduce its incidence. Size of gland. In all series reviewed in which this factor was taken into account, the incidence of post-treatment hypothyroidism was significantly greater in patients who had small glands, either nonpalpable or of normai size.7**’ The administered level of '*'I has been adjusted downward from the usual calculated amountin some Series in consideration of this factor.”* In the review of their surgical series, Nofal et al.” also described this feature, but normal sized glands were rarely operated on.’ This may be a point of bias in favor of surgery when attempting to compare ''I with surgery in relation to resultant hypothyroidism.'* Presence of nodules. The multinodular gland appearsto be moreresistant to treatment, whether it be radioiodine or surgery. It usually requires multiple doses of |] more frequently and has a much lower incidence of post-therapeutic hypothyroidism.?** Surprisingly, in patients with a single functioning nodule the incidence of subsequent hypothyroidism is greater.*’ In these patients it would ordinarily appear appropriate to give large doses since the nonnodular tissue would be relatively spared. However, the same results are found following surgery.” Therefore, the incidence of hypothyroidism must be related to something other than just the distribution of radioactivity in the gland following a therapeutic dose of "!I. Age of patient. There is a suggestion that the thyroid of younger individuals is more sensitive to radiation than that of older individuals and in someinstitutions the dose is reduced somewhat for the younger patient. The study of Segal et al.* showed a slightly higher incidence of myxedema in the younger patients. In some series the '*‘I-treated younger patients tended to have larger glands,’ whereasin other series these patients tended to be treated surgically. This factor can certainly bias results. Nofal et al.’ demonstrated a slightly higher incidence of post-therapy hypothyroidism in the younger age group with '*'I treatment, whereas the highest incidence following surgery wasin the patients who were between 40 and 49 years when treated. Size of dose. The size of the initial dose does appear to have an effect on subsequent development of hypothyroidism and has been the basis for several series using lower doses to control the disease.'*!*"!8 This may possibly slow the rate of the late cumulative incidence, but it certainly reduces the incidence of hypothyroidism in the first year. It does result in an increased requirement for multiple doses. an ae) » Pad af CHG Number of doses. There is an inverse relationship between numberof doses (and total doses) and the development of late-onset hypothyroidism in most studies.?*7-14 Usually patients requiring multiple doses had multinodular glands. In addition, response to the first dose selected out those patients whose glands were more radiosensitive.