The stimulating effect of thyroid secretion on growth and metabolism, shown both experimentally and clinically in the therapeutic triumphs of thyroid treatment in myxoedema and hypothyroidism, seems to justify the view that failure of thyroid function plays an important part in reducing the capacity for multiplication and regeneration of cells. Between the manifestations of hypothyroidism and those of old age there are undoubtedly striking resemblances, such as the dry skin, the loss of hair, the diminished mental and bodily energy, and the increased amount of interstitial fibrous tissue. In some respects myxoedema and cretinism imitate the senile state; myxoedema has indeed been described as more than a simulation and as the best example of secondary senilism (Hastings Gilford[100]); but this is a very different condition from healthy old age which does not present the picture of complete loss of thyroid function; old age therefore cannot be regarded as due to athyroidism. Usually atrophy of the thyroid accompanies and corresponds to that of the senescent body as a whole. No doubt pathological changes may occur in the thyroid, especially in women, during advancing years, and it is in such instances that thyroid feeding produces so much improvement that it has revived in our day the hope of an elixir of life and perpetual youth. It is not improbable that the administration of thyroid gland extract may, by preventing high blood pressure, delay or obviate the onset of arteriosclerosis, and so stave off senility due to this cause, but this is pathological old age and not the physiological involution.