The acute response of the rat thyroid gland to head-neck X-irradiation at the maximal tumor inductive dose is mainly manifested by a depression of serum thyroxine level. This acute disturbance normalizes within 8 weeks after X-irradiation. In unirradiated animals, thyroid necrotic changes appear that are age related. In head-neck X-irrad1ated rats, however, such changes show an apparent peak at one week after treatment. Thereafter, it subsides to the level indistinguishable from the controls. Except for an enlarged nucleus that occurs at one year after X-irradiation, no other histologic features can distinguish radiation necrosis from spontaneous necrosis.
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