The thymus is largest and more active during the neonatal and pre-adolescent periods. By the early teens, the thymus begins to atrophy and thymic stroma is replaced by adipose tissue. Neveheless, residual T lymphopoiesis continues throughout adult life. The thyroid reaches maximum weight ( 20-37 grams) by the time of pubey. Age Mass Bih about 15 gms Pubey About 35 gms Twenty- five years 25 gms Sixty years < 15 gms Seventy years as low as 5 gms