Human Physiology

Lesson 37 : Hypothalamus, Pitutary & Thyroid

Anterior Pituitary/Adenohypophysis

ANTERIOR PITUITARY/ADENOHYPOPHYSIS: uitary is one of the most important and critical endocrine gland in the human body and is responsible for secretion of a wide array of hormones that control virtually all the functions of body directly or indirectly. It is located in depression of sphenoid bone called sella tursica at base of the brain. It is about 0.5 to l g in weight with 1 cm diameter. Hypophysis is connected to hypothalamus by a hypophyseal / pituitary stalk.

Physiologically pituitary has 2 parts – Anterior & Posterior:

Anterior Pituitary/ Adenohypophysis	Posterior Pituitary/ Neurohypophysis
Hormones secreted are: Follicular Stimulating Hormone (FSH), Leutinizing Hormone (LH) Prolactin (PRL), Somototropic Hormone (STH) also called Growth Hormone (GH), Thyroid Stimulating Hormone (TSH), Adrenocorticotropic Hormone (ACTH) Melanocyte Stimulating Hormone (MSH)	Hormones secreted are: ADH (Vasopressin) Oxytocin These two hormones are synthesized inhypothalamus but secreted from neurohypophysis)

Gonadotropic Hormones: FSH and LH are together called gonadotropic hormones as they stimulate growth of gonads (ovaries and testes). They are secreted from anterior pituitary.

Follicle Stimulating Hormone (FSH): FSH is having two chains; ? (89 amino acids) and ? (115 amino acids) with a molecular weight of 32000 Daltons. Half life is 2 to 4 hours, pituitary output under the control of hypothalamic control & feedback mechanism involving gonadal hormone. FSH causes ovarian stimulation in females there by growth of ovarian follicles, maturation of oocytes within ovarian follicles, secretion of estrogen by components of graffian follicle. In males, FSH helps in initiation of spermatogenesis sequence during puberty.

Leutinizing Hormone (LH): LH similar structure as that of FSH but amino acid sequencing is different. In females the LH is responsible for causing the ovulation and release of ova from follicle. Subsequently affecting the granulose cells and brining about their leutinization and progesterone synthesis.

Prolactin (PRL):- PRL has 198 amino acid with a molecular weight of 23000 Daltons. It stimulates lactation in mammals.

Somatotropin (STH) or Growth Hormone: STH has a molecular weight of 22,000 Daltons with a chain length of 191 amino acids. Balance in STH secretion is achieved through balance between GHRH & GHIH (Somatostatin). Low blood glucose is stimulatory for release of GHRH from hypothalamus. Also stress, exercise, high protein intake and sleep increases STH secretion. STH is released at a near constant rate through out life. Main function of GH/STH is to cause growth through its anabolic action. Skeletal growth stops after puberty but it maintains its biological role as anabolic agent and therefore causes thickening of the bones later in life. It enhances the action of ACTH, TSH, LH and FSH on their target organs,

Physiological functions-

1. Protein synthesis in all cells of body.
2. Mobilization of fatty acids from adipose tissue & use of fatty acids for energy.
3. Use of glucose utilization i. e, increasing glucose in blood.

Growth hormone work through small proteins called somatomedins. Four types of somatomedins are present out of which somatomedin-c is most important and has molecular weight of 7500 Daltons. This molecule is produced by hepatocytes and also other cells.

Abnormalities due to GH/STH deficiency or over secretion:

Dwarfism: Dwarfism may result due to deficiency of GH secretion. The individual is not able to grow at the normal rate, therefore appear to have stunted growth. Such individuals usually have a short height with thick bones

Gigantism –GH is secreted at high rate and the individual becomes a giant. Such giants usually have been reported to be also suffering from diabetes mellitus.

Acromegaly: when cells of pituitary secreting GH go in overdrive after the patient has acquired adolescence, so he/she can’t grow tall as the bone shafts have merged with epiphysis, but it grows laterally and patient acquire weight, bones go thick and soft tissue.

Thyroid stimulating Hormone / Thyrotrophic hormone (TSH): TSH is a glycoprotein of molecular weight of 30,000 Daltons, having two chains ? (89 amino acids) and ? (112 amino acids). TSH is produced from anterior pituitary under TRH influence from hypothalamus. TSH affects the chief cells of thyroid gland and increases the production of T3 & T4 from thyroid gland through increasing the uptake of iodide, production and release of thyroxin or its analogue and increased proteolysis of thyroglobulin.

Adrenocortico Tropic Hormone (ACTH): It is a polypeptide of 39 amino acids and molecular weight of 4570. Secretion of ACTH is controlled through feedback mechanism from adrenal steroids at hypothalamic level. Also stress like hemorrhage, temperature, toxins, emotions affects CRH release to affect ACTH. Diurnal fluctuation is also seen with a higher level during early morning hours. ACTH stimulates adrenal cortex for secretion of cortisol/ corticosterone. The effect is less on release of mineralocorticoids from adrenals.

Melanocyte Stimulating Hormone (MSH): The amino acid chain length is 24 with a molecular weight of 1823 Daltons. MSH stimulates melanocytes (present between dermis and epidermis,) to form melanin and to disperse it in cells of epidermis. A high level of MSH for 8-10 days leads to darkening of the skin. In lower vertebrates, intermediate lobe of pituitary – pars intermedia is active & large amount of MSH is secreted. This secretion is independently controlled by hypothalamus in response to light exposure (MSH- IF & MSH- RF).