Thyroid Gland. The thyroid gland is composed of two lobes which are located on either side of the trachea. Both the lobes are interconnected with a thin flap of connective tissue called isthmus. The thyroid gland is composed of follicles and stromal tissues. Each thyroid follicle is composed of follicular cells, enclosing a cavity.
These follicular cells synthesise two hormones, tetraiodothyronine or thyroxine (T₄) and triiodothyronine (T₃). Iodine is essential for the normal rate of hormone synthesis in the thyroid. Deficiency of iodine in our diet results in hypothyroidism and enlargement of the thyroid gland, commonly called goitre. Hypothyroidism during pregnancy causes defective development and maturation of the growing baby leading to stunted growth (cretinism), mental retardation, low intelligence quotient, abnormal skin, deaf-mutism, etc. In adult women, hypothyroidism may cause menstrual cycle to become irregular. Due to cancer of the thyroid gland or due to development of nodules of the thyroid glands, the rate of synthesis and secretion of the thyroid hormones is increased to abnormal high levels leading to a condition called hyperthyroidism which adversely affects the body physiology.
Functions of Thyroid. Thyroid hormones play an important role in the regulation of the basal metabolic rate. These hormones also support the process of red blood cell formation. Thyroid hormones control the metabolism of carbohydrates, proteins and fats. Maintenance of water and electrolyte balance is also influenced by thyroid hormones. Thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT)which regulates the blood calcium levels

The Pituitary Gland. The pituitary gland is located in a bony cavity called sella tursica and is attached to hypothalamus by a stalk. It is divided anatomically into an adenohypophysis and a neurohypophysis. Adenohypophysis consists of two portions, pars distalis and pars intermedia.
The pars distalis region of pituitary, commonly called anterior pituitary, produces following hormones.
a. Growth hormone (GH). Over-secretion of GH stimulates abnormal growth of the body leading to gigantism and low secretion of GH results in stunted growth resulting in pituitary dwarfism.
b. Prolactin (PRL). Prolactin regulates the growth of the mammary glands and formation of milk in them
c. Thyroid stimulating hormone (TSH). TSH stimulates the synthesis and secretion of thyroid hormones from the thyroid gland.
d. Adrenocorticotrophic hormone (ACTH).ACTH stimulates the synthesis and secretion of steroid hormones called glucocorticoids from the adrenal cortex.
e. Luteinizing hormone (LH). LH and FSH stimulate gonadal activity and hence are called gonadotrophins. In males, LH stimulates the synthesis and secretion of hormones called androgens from testis. In females, LH induces ovulation of fully mature follicles (graafian follicles) and maintains the corpus luteum, formed from the remnants of the graafian follicles after ovulation.
f. Follicle stimulating hormone (FSH). In males, FSH and androgens regulate spermatogenesis. FSH stimulates growth and development of the ovarian follicles in females. MSH acts on the melanocytes (melanin containing cells) and regulates pigmentation of the skin.
Pars intermedia secretes only one hormone called melanocyte stimulating hormone (MSH). However, in humans, the pars intermedia is almost merged with pars distalis.
g. Neurohypophysis (pars nervosa) also known as posterior pituitary, stores and releases two hormones called oxytocin and vasopressin, which are actually synthesised by the hypothalamus and are transported axonally to neurohypophysis. Oxytocin acts on the smooth muscles of our body and stimulates their contraction. In females, it stimulates a vigorous contraction of uterus at the time of child birth, and milk ejection from the mammary gland. Vasopressin acts mainly at the kidney and stimulates re-absorption of water and electrolytes by the distal tubules and thereby reduces loss of water through urine (diuresis). Hence, it is also called as anti-diuretic hormone (ADH).

(a) Function of Parathyroid Hormone. Parathyroid hormone (PTH)increases the Ca²⁺ levels in the blood. PTH acts on bones and stimulates the process of bone resorption (dissolution/ demineralisation). PTH also stimulates reabsorption of Ca²⁺ by the renal tubules and increases Ca²⁺ absorption from the digested food.It is, thus, clear that PTH is a hypercalcemic hormone, i.e., it increases the blood Ca²⁺ levels. Along with TCT, it plays a significant role in calcium balance in the body.
(b) Functions of Thyroid Hormones. Thyroid hormones play an important role in the regulation of the basal metabolic rate. These hormones also support the process of red blood cell formation. Thyroid hormones control the metabolism of carbohydrates, proteins and fats. Maintenance of water and electrolyte balance is also influenced by thyroid hormones. Thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT)which regulates the blood calcium levels.
(c) Functions of Thymosins. Thymosins playa major role in the differentiation of T-Iymphocytes, which provide cell-mediated immunity. In addition, thymosins also promote production of antibodies to provide humoral immunity.
(d) Androgens. Development of male accessory sex organs, spermatogenesis
(e) Estrogens. Development of female accessory organs, mammary glands, ovarian follicle
(f) Insulin and Glucagon. Insulin maintains glucose homeostasis and glucagon maintains normal glucose level in blood.

(a) Hypothalamus - Releasing hormone and Inhibiting hormone.
(b) Pituitary - Growth hormone, Prolactin, Thyroid stimulating hormone, Adrenocorticotrophic hormone, Luteinizing hormone, Follicle stimulating hormone, Melatonin.
(c) Thyroid - Tetraiodothyronine, Triiodothyronine.
(d) Parathyroid - Parathyroid hormone.
(e) Adrenal -  Adrenalin or Epinephrine, Noradrenaline or norepinephrine Corticoids.
(f) Pancreas - Glucagon, Insulin.
(g) Testis - Testosterone.
(h) Ovary - Estrogen, Progesterone.
(i) Thymus - Thymosin.
(j) Atrium - Atrial Natriuretic Factor
(k) Kidney - Erythropoietin.
(l) GI Tract - Gastrin, Secretin, Cholecystokinin (CCK)Gastric Inhibitory Peptide (GIP)

The endocrine glands and hormones that are responsible for maintaining calcium homeostasis, are thyroid and parathyroid glands and their associated hormones an calcitonin and Parathyroid Hormone (PTH).
(i) Parathyroid glands are the glands developed from the endoderm of the embryo The cells of parathyroid glands are of two types, i.e., chief cells and oxyphil cells. Thr chief cells of the parathyroid glands secrete parathyroid hormone (PTH).
This hormone (PTH) is involved in regulating calcium and phosphate balance betweer the blood and other tissue. It mobilises the release of calcium into the blood from bones. PTH increases calcium reabsorption by the body organs like intestine anc kidneys.
(ii) Thyroid gland is the largest endocrine gland located anterior to the thyroid cartilage o. the larynx in the neck. This gland plays a vital role in maintaining calcium homeostasis It releases thyroculcitonin hormone produced by the parafollicular cells, also called 'C cells.
This hormone is secreted when the calcium level in blood gets high. It is a 32 aminc acid peptide hormone that lowers the calcium level by suppressing release of catciurr ions from the bones. Thus, calcitonin has an action opposite to that of the parathyroic hormone on calcium homeostasis.

(1) Melanocytes stimulating –release hormone. Stimulate the function of MSH.
(2) Melanocyte stimulating-inhibiting hormone. It inhibits the release of MSH.

Characters which do not play direct role in sexual reproduction but are basically means of sexual differentiation are called secondary sexual characters. For example, facial hair and deep voice in males and thin voice in females are secondary sexual characters

Adrenaline and noradrenaline are rapidly secreted in response to stress of any kind and during emergency situations and are called emergency hormones or hormones of Fight or Flight. These hormones increase alertness, pupilary dilation, piloerection (raising of hairs), sweating etc. Both the hormones increase the heart beat, the strength of heart contraction and the rate of respiration. Finally the body is ready to counter the emergency situations.

Parathyroid hormone (PTH) increases the Ca2+ levels in the blood. PTH acts on bones and stimulates the process of bone resorption (dissolution/ demineralisation). PTH also stimulates reabsorption of Ca²⁺ by the renal tubules and increases Ca²⁺ absorption from the digested food. It is, thus, clear that PTH is a hypercalcemic hormone, i.e., it increases the blood Ca²⁺ levels. Along with TCT, it plays a significant role in calcium balance in the body

Endocrine glands are tubeless glands and their secretion is directly released on the target site. Because of the absence of tubes the hormone is secreted in the gland itself hence the name endocrine gland is given for them.