Invited Review: A patient with primary hypothyroidism was given thyrotropin releasing hormone (TRH) intravenously prior to commencement of replacement therapy and again while receiving 25, 50 and 100 micrograms of 1-thyroxine per day. Initially, basal levels of thyrotropin were markedly elevated and rose dramatically after TRH administration. With increasing doses of thyroxine, basal thyrotropin levels fell and response to TRH progressively diminished. Basal triiodothyronine (T3) levels were low on each occasion and failed to rise in response to TRH. However, the serum T3level when the patient was receiving 100 micrograms of thyroxine per day was 20 ng/100 ml higher than before therapy, probably as a result of peripheral conversion of thyroxine to triiodothyronine.