(a) Definition: Hormones (Gk. hormein— to excite) are chemical messengers or informational molecules produced by ductless glands which are translocated by circulatory system to other body parts for inducing and coordinating their activities including growth. First hormone, secretin, was discovered by Bayliss and Starling (1902). The term hormone was coined by Starling (1905)

(b) Characteristics:

• Hormones are produced by endocrine or ductless glands.
• They are poured into circulatory system for passage to different body parts.
• Target Sites. Hormones act on specific cells, tissues and organs called target sites, generally away from the place of their synthesis.
• They function as chemical messengers or informational molecules that trigger specific chemical and physiological processes of target cells.
• Slow Action. Since hormones reach the target sites through blood, their effect appears after a lag period. They are slow acting with the exception of adrenaline.
• Chemical Nature. Hormones are small sized organic molecules which are of diverse origin— proteins, peptides, amino acids, amines and steroids.
• Non-nutrient Nature. Hormones are nonnutrient in nature. They have no role in providing energy or body building materials. Hormones take part in stimulation or inhibition of physiological processes.
• The hormones are effective in very low concentration, e.g., adrenaline one in 300 million parts.
• It is very specific. TSH acts only on thyroid while thyroxine affects all body parts.
• Hormones are generally produced in response to specific stimuli.
• Hormones are ultimately broken down or consumed during their activity in target cells.
• Deficiency or Excess. Both deficiency and excess of hormone are harmful, often leading to serious disorders.

(c) The Hormone are Required:

• Functioning of Mammary Glands. Prolactin.
• Regulation of Calcium and Phosphate in Blood. Parathormone.
• Lowering of Blood Glucose. Insulin.

· Development of Moustache and Beard in Human Male. Testosterone.