Jet lag is caused by the disruption of the body clock as it is out of synchronisation because of the unfamiliar time zone of the destination. The body experiences different patterns of light and dark conditions than it is normally used to, which disrupts the natural sleep-wake cycle.

Melatonin is a hormone that plays a key role in body rhythms and causes jet lag. After the sun sets, eyes perceive darkness and alert the hypothalamus to begin releasing melatonin, which promotes sleep. Conversely, when the eyes perceive sunlight, they tell the hypothalamus to withhold melatonin production.

However, the hypothalamus can not readjust its schedule instantly and it may take several days, to overcome this problem.

Glucocorticoids, particularly cortisol, produce anti-inflammatory reactions and suppress the immune response. The source for glucocorticoids is the middle zone, which is the widest of three zones, in adrenal cortex called zona fasciculata.

The functions of glucocorticoids as the name suggests they is that they affect carbohydrate metabolism and metabolism of proteins and fats. They stimulate gluconeogenesis, lipolysis and proteolysis.

They also inhibit cellular uptake and utilisation of amino acids. Cortisol is also called stress hormone as it copes with stress.

The thymus gland is a lobular structure located on the dorsal side of the heart and the aorta. It is derived from the endoderm of the embryo. Thymus secretes a hormone named thymosin which stimulates the development of White Blood Cells (WBCs), involved in producing immunity.

Thymus is degenerated in old individuals, resulting in decreased production of thymosin. As result the immune system becomes weak, in old people.

Hypothyroidism during pregnancy cause defective development and maturation of the growing baby leading to a stunted growth (cretinism), mental retardation, low intelligence Quotient (IQ), abnormal skin, deafmutism, etc.

Differences between hypothyroidism and hyperthyroidism are as follows

Hypothyroidiswm	Hyperthyroidism
It is the insufficient secretion of thyroid hormones causing cretinism in children and myxoedema in adults	This syndrome arises as the body tissues and are exposed to excessive levels of $T_3$ and $T_4$. The main effects are due to increased metabolic rate. The gland enlarges and may develop single or multiple hormone secreting cells such as in Grave's disease and toxic nodular goitre
Cretinism is associated with the retarded mental and physical development. The child receives hormones from the mother before birth, so appears normal at first, but within a few weeks or months, it becomes evident that physical and mental development is retarded. Symptoms are disproportionately short limbs, a large protruding tongue, coarse dry skin, poor abdominal muscle tone and an umbilical harnia.	Exophthalmic goitre (Grave's disease) is the most common cause solid of thyrotoxicosis in adults. It affects, women more commonly than men. High levels of thyroxine are secreted, which are not subjected to the normal secretion of TRH from the hypothalamus and TSH from the anterior pituitary. Exophthalamus (Protrusion of the eyeballs) due to the deposition of excess fat and fibrous tissue behind the eyes, is often present in Grave's disease. In severe cases, the eyelids may not completely cover the eyes during blinking of the conjuctiva and thus, predisposed to infection.
Myxoedema (Gull's disease), this conditions is common in adults and five times more common in females than in males. It results in an abnormally low metabolic rate and lack of response to demand for increased energy, by muscles, during exercise. Mental and physical processes become slower, skin become coarse, the hair lacks lustre, becomes brittle and tends to fall out.	Toxic nodular goitre (Plummer's disease) is associated with excess secretion of thyroxine leading to the general effects of increased metabolic rate.