Final answer:
The lumen of each thyroid follicle is filled with a substance called colloid, which is a gel-like mass containing thyroglobulin and bound thyroid hormones used to store these hormones until they are needed.
Step-by-step explanation:
The substance that fills the lumen of each follicle in the thyroid gland is primarily composed of a gel-like mass, which chiefly consists of thyroglobulin and bound thyroid hormones. The correct answer to this question is b) Colloid.
Colloid serves as a storage medium for the precursors of thyroid hormones within the spherical thyroid follicles. The thyroid follicles, which are lined with a simple cuboidal epithelium, synthesize and store thyroid hormones. The follicles are uniquely tailored to absorb iodine, which is vital for the production of thyroid hormones. The thyroid follicles produce hormones such as T3 and T4 which can be stored in the colloid or released into the bloodstream as necessary in response to the body's metabolic demands.
The parafollicular cells, or C cells, located among the follicles also produce the hormone calcitonin. This sophisticated system ensures that the thyroid can regulate the body's metabolism and calcium balance effectively. Understand that colloid is crucial in this endocrine process as it acts as a reservoir for the inactive forms of the hormones until they are needed.