Can you identify the thyroid follicles, colloid and septa in this low power image of the thyroid?
Follicular cells - are almost columnar in appearance in some regions, whilst elsewhere they have a low cuboidal appearance.
This is because in active glands, the follicles are smaller, and have reduced colloid - the cuboidal lining cells are relatively tall because they are actively making and secreting hormones - so packed full of ER and golgi.
The thyroid gland is unusual, in that the hormones are stored in cavities, surrounded by secretory cells, which make up a 'follicle'. To secrete the hormone, the hormone is re-absorbed from the cavity, and then released into the surrounding interstitial spaces. The stored hormone is bound to a glycoprotein, and this stored hormone is called 'colloid'.
This gland secretes iodine containing hormones called Tri-iodo thyronine (T3) and thyroxine (T4) of which T3 is more active. It regulates the basal metabolic rate, and it is regulated by the pituitary hormone TSH. It also secretes calcitonin - which regulates blood calcium levels. Secretion of calcitonin causes blood calcium levels to drop, and its secretion is directly dependent on blood calcium levels.
Colloid is an inactive precursor of T3 and T4. It is made up of a glycoprotein called thyroglobulin, made by the epithelial cells, which is bound to iodine. The iodine binds to the tyrosine residues of thyroglobulin.