Which of the following is an endocrine gland?

• A. Thyroid gland
• B. Heart
• C. Lymph node
• D. Skin

Answer: (A)

Endocrine glands release hormones directly into the bloodstream to regulate distant targets. The thyroid gland fits this role because its cells produce thyroid hormones that travel through the blood to modulate metabolism, growth, and development. The heart is mainly a muscular pump, though it does secrete a hormone in some contexts, its primary function isn’t as an endocrine gland. Lymph nodes are immune organs that filter lymph and support immune responses, not hormone secretion. The skin functions mainly as a protective barrier and participates in other non-endocrine activities, not as a primary hormone-secreting gland. Because it is specialized for producing and releasing hormones into the bloodstream, the thyroid gland is the endocrine gland.