The secretory components of the gland are made of quasi-spherical structures called follicles, which are surrounded by secretory cells. The lumen of the follicles fills with a thick substance called colloid, made out of large proteins containing thyroid hormones. The colloid serves as a reservoir for Thyroid hormones (TH). Within the follicles of the thyroid gland other endocrine cells, called the parafollicular cells or C are located. These are responsible for the production of Calcitonin, a hormone involved in the regulation of calcium metabolism