The thymus gland is a vital organ for the immune system, located in the chest cavity, behind the sternum (breastbone), in front of the trachea, below the thyroid gland, and in front of the upper part of the heart. Also known colloquially as "sweetbread" (uykuluk in Turkish context), this gland is a small, encapsulated, pinkish-gray structure resembling the letter H. The thymus gland reaches its largest size at birth but begins to shrink during puberty, becoming almost invisible in adulthood. In elderly adults, its weight is estimated to be approximately 5 grams. In boys, its size may be slightly larger than in girls, and it can be detected in X-ray images in children up to 3 years of age. Although its size diminishes after puberty, the thymus gland continues to perform its important functions throughout life.