(i) The hormones are chemical messengers released into the blood and circulated as chemical signals.
(ii) They influence the metabolic activities of the body by means of hormones.
(iii) Deficiency or excess secretion of hormones will lead to disorders.
(iv) Thus hormones act as chemical messenger and coordinate different physical, physiological mental activities and maintain homeostasis.

1. The butterfly shaped thyroid gland is a bilobed gland located below the larynx on each side of upper trachea. It is the largest endocrine gland in the body.
2. Its two lateral lobes are connected by a median tissue mass called isthmus.
3. Each lobe is made up of many lobules. The lobules consist of follicles called acini (acinus In singular).
4. Each acinus is lined with glandular, cuboidal or squamous epithelial cells which is filled with thick glycoprotein mixture consist of thyroglobulin molecules.
5. This acini of thyroid gland are often called as major metabolic hormones.
6. The follicular cells of thyroid gland secretes two hormones they are tri-iodothyronine (T₃) and thyroxine [tetra-iodothyronine] (T₄).
7. The parafollicular cells or 'C' of thyroid gland secretes a hormone called thyrocalcitonin.
8. To produce thyroid hormones Iodine is very essential.

(i) In human, the pineal gland is situated behind the third ventricle of brain.
(ii) It is otherwise known as cerebri or conarium and formed of parenchymal cells and interstitial cells.
(iii) It secretes the hormone, melatonin, which plays a central role in the regulation of circadian rhythm of our body and maintains the normal sleep wake cycle.
(iv) It also regulates the timing of sexual maturation of gonads.
(v) In addition to this it play a important role in metabolism, pigmentation, menstrual cycle and defense mechanism of our body.

Somatotropic hormone is important for normal growth because it performs several duties like.
(i) It promotes growth of all the tissues and metabolic process of the body.
(ii) It promotes the metabolism of Carbohydrates, proteins and lipids and increases the rate of protein biosynthesis in the cells.
(iii) It increases the release of fatty acids from adipose tissue and decreases the rate of glucose utilization for energy by the cells. It helps to minerals retention in the body like nitrogen, potassium, phosphorous and sodium.
(iv) The hyper secretion and hypo secretion of growth hormones leads to several disorders like dwarfism, gigantism and acromegaly.

Cretinism is due lower secretion of growth hormone in infants; [ie] hypothyroidism.
Symptoms of Cretinism:
In infants, hypothyroidism causes cretinism .A cretin shows
1. Retarded skeletal growth
2. Absence of sexual maturity.
3. Retarded mental ability.
4. Thick wrinkled skin.
5. Protruded enlarged tongue.
6. Bloated face.
7. Thick and short limbs occurs