Which of the endocrine glands is capable storing it hormones extracellularly and then slowly releasing them?

The endocrine gland that is capable of storing its hormones extracellularly and then slowly releasing them is the thyroid gland.

The thyroid gland stores its hormones, thyroxine (T4) and triiodothyronine (T3), in the form of thyroglobulin, a large protein molecule. Thyroglobulin is synthesized and released from the thyroid cells into the follicular lumen, where it is stored. When the body needs thyroid hormones, the follicular cells take up thyroglobulin by endocytosis, degrade it, and release the T4 and T3 into the bloodstream.

The ability to store hormones extracellularly allows the thyroid gland to maintain a relatively constant level of thyroid hormones in the blood, even when the demand for these hormones fluctuates.