Final answer:
The true endocrine gland in the thyroid gland that uses a portal system is the pituitary gland. It produces and releases several hormones that regulate the function of other endocrine glands.
Step-by-step explanation:
The true endocrine gland in the thyroid gland that uses a portal system is the pituitary gland. The pituitary gland is often referred to as the 'master gland' because it produces and releases several hormones that regulate the function of other endocrine glands in the body. One of the hormones secreted by the pituitary gland, called the thyroid-stimulating hormone (TSH), directly affects the thyroid gland's activity.