An important organ of the endocrine system .
Follicular cells make up most of the gland .
The space inside the follicles is filled with a semifluid , colloid material that is essential for the production of T 4 and T 3 . Parafollicular cells occur singly or in small groups in the spaces between the follicles. They secrete the hormone calcitonin . Also between the follicles are numerous blood capillaries , small lymphatic vessels , and connective tissue . The entire thyroid gland is encased in a thin outer layer of connective tissue.