The effect of thyroglobulin immunization on thyroxine secretion rate and peripheral utilization of thyroid hormones has been studied, along with morphologic changes in the thyroid, adrenals and hypophysis. Thyroglobulin immunization decreased the thyroxine secretion rate as well as the disappearance rate of thyroid hormones from the blood. The protein-bound iodine was also slightly elevated in immunized animals. A significant increase in the weight of the pituitary, adrenals and thyroid was noted in immunized animals. The thyroid glands of immunized animals showed an interstitial inflammatory cell reaction characteristic of chronic thyroiditis. The increase in weight of the adrenals was associated with an increase in the size of cells of the zona fasciculata, whereas the increase in pituitary weight was associated with cellular degeneration and replacement by nonfunctional connective tissue. Possible explanations for the anomalous situation of decreased peripheral utilization of thyroid hormones and decreased TSH secretion have been presented.