Help me understand how the thyroid secretes Please have a look at this close up light microscopic image of a thyroid gland: !enter image description here So I'm guessing the cubic epithelial cells surrounding this big red pulp are the secreting cells...? If so, is the red pulp a blood vessel that they are secreting into? Am I misunderstanding?

That "red pulp" spaces you're referring are not blood vessels but, in fact, thyroid follicles that selectively absorb iodine from the blood for production of thyroid hormones and storage of thyroglobuline. So, they act as a reservoir of the materials used by the thyroid's epithelial cells, also called follicular cells, to produce thyroid hormones (T3 and T4).