Which gland is the primary source of melatonin?

• A. Pituitary gland
• B. Pineal gland
• C. Hypothalamus
• D. Thymus

Answer: (B)

Melatonin production is centered in the pineal gland, which acts as the body's main source of this hormone. The pineal gland converts serotonin into melatonin, and its activity follows a circadian pattern: levels rise in darkness and fall with light. This rhythm helps regulate sleep-wake cycles. The light-dark signal reaches the pineal gland indirectly through the retina and the brain’s circadian center, the suprachiasmatic nucleus, which modulates the autonomic input to the pineal gland to increase melatonin at night and decrease it during the day. Other glands mentioned have different roles—pituitary secretes various hormones for growth and metabolism, the hypothalamus coordinates endocrine and autonomic function but does not supply melatonin, and the thymus is involved in immune development. So, the pineal gland is the primary source of melatonin.