How are Hormones secreted ?

I was asked by one of my patients that how are Hormones secreted? Below I am explaining in detail what are the mechanisms involved in the secretion of hormones.

A) Feedback Mechanisms:

The basic function of a hormone is to regulate the metabolic activity of its target cells and to maintain this function the endo­crine gland that secretes the hormone must receive constant rapid information (feedback) on the state of the systems regulated.

1) Direct negative feedback:

The rate of release of the hormone depends on the concentration of the circulating chemical sub­stance which it controls e.g. insulin decreases the blood glucose levels and a change in blood glu­cose concentration alters the rate of secretion of insulin.

2) Indirect negative feedback:

The hypothalamus is indirectly involved in the regulation of hormone secretion from the endo­crine gland by controlling the release of the appropriate adenohypophyseal hormones, e.g. the blood levels of thyroxine regulates the secretion of TSH from hypothalamus which in turn regulates the secretion of TSH from adenohypophyses which normally stimulates the thyroid to secrete thyroxine.

3) Positive feedback:

The change elicited by the action of the hormone alters the rate of release of that hormone or some other hormone so that its effect is further en­hanced. This type of feedback could rapidly become uncontrollable if there were no other con­trol mechanisms, e.g. oestrogen secretion from the ovarian follicle is controlled by adenohypophyseal hormone FSH. At a particular concentration, oes­trogen stimulates the release of more FSH.

B) CNS Influence :

The central nervous system plays a role in the spontaneous secretion of and stress induced secretion of hormones. This is me­diated through the hypothalamus and pituitary. The role of CNS is seen in events around puberty, in the increase in growth hormone during REM sleep and in the release of growth hormone and ACTH with stress.

BIORHYTHM:

Hormones are released in a rhythmic manner. This can vary:

1. Over minutes to hours: e.g. pulsatile secretion ofLH and testosterone.
2. Daily: e.g. circadian rhythm of Cortisol secre­tion.
3. Weekly: e.g. menstrual cycle.
4. Seasonal: e.g. thyroxine.

Sleep also influences hormonal release, e.g. go- nadotrophins in prepubertal stage.