Thyroid follicles, especially of their irregular types, are studied throughout the life cycle of female fowl, i.e. the pre-laying, the first laying, the laying, the broody and the molting stages. The results obtained are as follows. 1. In the thyroids, except for the broody stage, the author found peculiar "irregular-shaped follicles" consist of flattened epithelia (figs. 2-10). The colloids contained in these follicles are stained faintly with eosin dye. 2. From their shapes and stainability, the "irregular-shaped follicles" have many transitional types (figs. 11-17). 3. The above-described facts suggest that the "irregular-shaped follicles" indicate the various stages of release of the active thyroid hormone into the capillaries. 4. These process are most found in the thyroids of laying and molting stages (first and middle), while in the broody and the last molting stages they are decreased in number.