When TSH is injected into chicks with I131-labeled thyroids there is a concomitant depletion of I131, I127 and sialic acid from the thyroid gland during about 24 hr, presumably as components of thyroglobulin. The ratio of iodine to sialic acid in the material lost appears to be independent of the TSH dosage. However, the ratio of iodine to sialic acid changes with time during repletion after the thyroid has been depleted by TSH treatment. The data suggest that, under fasting conditions, the sialic acid content of the thyroid returns to normal levels more rapidly than does the iodine content.