T4 thyrotoxicosis with normal or low serum T3 concentration.

Eight patients are described with elevated plasma thyroxine (T4) concentrations, but normal or low plasma triiodothyronine (T3) concentrations by radioimmunoassay. All were suffering also from non-thyroidal illness, which in six cases was severe, and in two fatal. The two patients in reasonable general health were acromegalic. It is possible that in sick thyrotoxic patients there is an impairment of peripheral conversion of T4 to T3, similar to that occurring in sick euthyroid patients. However, further proof of thyrotoxicosis must be obtained in patients with high T4 and normal or low T3 levels by studying the response of the pituitary to TRH. Meanwhile, in the sick or elderly, estimation of serum or plasma T3 concentration may not be a good screening test for thyrotoxicosis.