Prealbumin associated hyperthyroxinaemia represents a rare but important protein binding anomaly, causing difficulties in differential diagnostics. This anomaly is based on a variant prealbumin showing an increased affinity to T4 and rT3 and in some cases also when using T4 analogue tracers. The condition of the patient is clinically euthyroid which is confirmed by a normal bTSH level, a normal response to TRH, and peripheral parameters (BMR, ASR, STI, SHBG) in the euthyroid range. This paper describes the diagnostic difficulties in a 45-year-old male pointing out the significance of precise laboratory determination of prealbumin associated hyperthyroxinaemia in order to avoid misdiagnosis and wrong treatment.