The pulse wave arrival time (designated QKd), has been further demonstrated to be useful in evaluating thyroid function under conditions of "diminished thyroid reserve", and in the evaluation of the changes in thyroid hormones which occur during hypocaloric feeding in humans. The QKd has been used to demonstrate that supplementation with oral thyroxin results in improvement at the target organ level, in patients with diminished thyroid reserve as manifested by low or borderline low thyroid function with moderate or minimal TSH elevation. In human subjects undergoing prolonged hypocaloric feeding, normally there is a decrease in serum T4 and T3, to maintain a euthyroid level of T3, fails to provide complete return to normal of the QKd. These findings suggest that QKd is more closely observed in peripheral thyroid hormones levels during hypocaloric feeding is indeed an adaptive mechanism which results in a state of hypometabolism.