Protein concentration of the rat thyroid colloid during thyroxine treatment

Sprague-Dawley rats were given daily doses of 20 mug thyroxine for varying periods between 1 and 21 days. Samples of colloid were obtained by micropuncture in vivo of follicles in the thyroid isthmus. The protein content of the samples was determined by gel electrophoresis and the volume by a technique based on radioactivity measurement. The relative volume of the follicle lumina was determined by light microscopy. Treatment with thyroxine resulted in an increase in the protein concentration of the colloid. This increase was statistically significant after 2 days and the concentration remained elevated throughout the experimental period. The size of the follicle lumina also increased, most rapidly during the first week. It is suggested that the increase in protein concentration of the colloid during thyroxine treatment is due to inhibition of endocytosis of colloid in combination with the discharge into the follicle lumen of highly concentrated protein.