It has been our clinical observation that patients with hypothyroidism are relatively insensitive to the discomfort of venepuncture, whereas thyrotoxic patients seem to have a heightened sensitivity. In an initial study we have measured the sensory thresholds of perception along with motor responsiveness in hypothyroid, thyrotoxic and euthyroid subjects, employing a simple and readily reproducible technique. Sensory thresholds were elevated and motor responsiveness impaired in hypothyroid subjects as compared to euthyroid controls. In thyrotoxic subjects, motor responsiveness was significantly enhanced, but sensory thresholds did not differ from control values. In a subsequent study the threshold abnormalities of thyroid dysfunction were corrected by rendering the patients euthyroid with appropriate therapy. Warming a further group of untreated hypothyroid patients produced a similar improvement in motor responsiveness to that seen in the L-thyroxine-treated group, thus implying that this parameter is at least, in part, temperature dependent. Sensory thresholds would seem to be reliable reflectors of tissue thyroid status in hypothyroidism, whereas motor responsiveness seems the better guide to thyroid status across the whole spectrum of thyroid function.