AKR mice rendered hyperthyroid by thyroxine had a significantly lower incidence of spontaneous lymphatic leukemia than did AKR mice rendered hypothyroid by thiouracil. In this experiment, the hyperthyroid mice weighed less than the hypothyroid animals. However, when AKR mice were treated with thyroxine, but in addition were fed supplements of vitamins and liver to protect them against the toxic effects of the hormone, they had a higher incidence of spontaneous lymphatic leukemia than did AKR mice rendered hypothyroid by thiouracil and an iodine-deficient diet. In these experiments the hyperthyroid mice weighed more than the hypothyroid animals. Hence, the influence of hyper- or hypothyroidism on the incidence of spontaneous lymphatic leukemia of AKR mice depends on what effect these states have on the body weight.

Hyper- and hypothyroidism did not influence the survival time of AKR mice that were allowed to die “naturally,” nor did hyperthyroidism significantly alter the length of survival of AKR mice with a transplanted lymphatic leukemia.

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