Final answer:
The hypothalamus and pituitary gland work together in a feedback loop to regulate the production of thyroid hormones. Together they form a critical part of the endocrine system, with the pituitary gland often being referred to as the 'master gland' for its role in controlling other endocrine glands.
Step-by-step explanation:
The correct answer to the question of which of these work together in a feedback loop is D. Hypothalamus and pituitary gland. The hypothalamus and pituitary gland play a central role in the endocrine system's regulation of other glands. The hypothalamus, located in the brain, integrates signals from the body and other brain areas, initiating endocrine responses to environmental changes. It controls the pituitary gland, which produces hormones that regulate various bodily functions as well as stimulating other endocrine glands to release hormones. Numerous feedback loops involve these two structures, including the negative feedback loop that regulates the production of thyroid hormones, involving the hypothalamus, pituitary gland, and the thyroid gland.