SummaryWhereas routine blood biochemistry can give some indication towards the diagnosis of hyperthyroidism, non of these data can establish this diagnosis. If the basal metabolic rate is low or normal the diagnosis of hyperthyroidism can be rejected; the finding of a high value, however, does not prove the existence of thyroid hyperfunction. A decreased Achilles reflex time strongly supports the diagnosis of thyrotoxicosis, since such a decrease is never found in non-hyperthyroidism. However, more than half of thyrotoxic patients have a normal reflex time. The basal metabolic rate and Achilles reflex time have lost much of their importance since the direct measurement of blood hormone levels is now readily available.