Name four male sex traits caused by androgens. 35. The principle androgens are and androstenedione. Andro means Thymus The thymus is located: The thymus secretes the hormone involved in the immune response. that increases T-cell activity. T-cells are At what age is the thymus most active? Pineal The pineal is located: The pineal secretes the hormone when What affect would sunlight or artificial light have on melatonin secretion?