Relation between thyroid function and serum levels of long-acting thyroid stimulator.

Bioassays of long-acting thyroid stimulator (LATS) were performed in three groups of subjects: in normal controls, in thyrotoxic patients before and serially after 131-I treatment, and in patients with hypopituitarism. Of the untreated thyrotoxic patients, 27.7% had raised serum LATS levels initially. There was no correlation between the relapse rate after 131-I therapy and the initial or subsequent LATS titres: in particular, thyrotoxicosis sometimes recurred after an initially high LATS titre had fallen into the normal range. The distribution of the results of LATS assays in the hypopituitary patients was significantly different from that in the normal subjects; 4 out of 27 hypopituitary patients had LATS levels above the normal range, although they had no thyrotoxicosis. On the other hand, the majority of the patients with hypopituitarism, 19 out of 27, had LATS titres below the mean normal level, possibly due to deficiency of a substance we have termed pseudo LATS. These results raise doubts about a direct causative role for LATS in most cases of thyrotoxicosis.