The size distribution of thyroid follicles in single thyroid glands, a peculiar problem of particle size distribution in stereology.

Follicle sections vary in size according to their inclination mainly in hypertrophied thyroids. Sagittal sections elicit a shift to small classes, while transversal sections, to large classes. There is no mathematical transformation able to convert data obtained by sections into "true" sizes of follicles, as may be achieved by isolation. All papers emphasizing only section diameters and inferring about follicle size must be cautiously interpreted, and even discarded, if they tend to establish differences in follicle size solely on that basis. Follicle isolation is necessary if we need to establish follicle size and shape. Although proportions are almost stable for rats of the same age and weight, the proportions of follicle size may vary, as dependent on age and other internal and external conditions. The situation is most complicated by colloid accumulation; but despite these impedimenta, TSH secretion enlarges the follicles, a situation hitherto not well established. Cell enlargement is consequently followed by follicle enlargement, despite colloid resorption.