● The `color{brown}"pineal gland"` is located on the `color{violet}"dorsal side of forebrain"`.

● Pineal secretes a hormone called `color{brown}"melatonin"`.

● `color{violet}"Melatonin"` plays a very important role in the regulation of a `color{violet}"24-hour (diurnal) rhythm"` of our body.

● For example, it helps in maintaining the normal rhythms of `color{violet}"sleep-wake cycle"`, `color{violet}"body temperature"`.

● In addition, melatonin also influences `color{violet}"metabolism"`, `color{violet}"pigmentation"`, the `color{violet}"menstrual cycle"` as well as our `color{violet}"defense capability"`.

● The `color{brown}"thyroid gland"` is composed of `color{violet}"two lobes"` which are located on `color{violet}"either side of the trachea"`.

● Both the lobes are `color{violet}"interconnected"` with a thin flap of connective tissue called `color{brown}"isthmus"`.

● The thyroid gland is composed of `color{violet}"follicles"` and `color{violet}"stromal tissues"`.

● Each thyroid follicle is composed of `color{violet}"follicular cells"`, enclosing a `color{violet}"cavity"`.

● These follicular cells synthesise `color{violet}"two hormones"`, `color{brown}"tetraiodothyronine or thyroxine"` (`T_4`) and `color{brown}"triiodothyronine"` (`T_3`).

● `color{violet}"Iodine"` is essential for the `color{violet}"normal rate"` of hormone synthesis in the thyroid.

● `color{violet}"Deficiency of iodine"` in our diet results in `color{violet}"hypothyroidism"` and enlargement of the thyroid gland, commonly called `color{brown}"goitre"`.

● `color{brown}"Hypothyroidism"` during pregnancy causes `color{violet}"defective development and maturation"` of the growing baby leading to `color{violet}"stunted growth (cretinism)"`, `color{violet}"mental retardation"`, `color{violet}"low intelligence quotient"`, `color{violet}"abnormal skin"`, `color{violet}"deaf-mutism"`, etc.

● In adult women, hypothyroidism may cause `color{violet}"menstrual cycle"` to become irregular.

● Due to `color{violet}"cancer of the thyroid gland"` or due to development of `color{violet}"nodules of the thyroid glands"`, the rate of synthesis and secretion of the thyroid hormones is increased to `color{violet}"abnormal high levels"` leading to a condition called `color{brown}"hyperthyroidism"` which adversely affects the body physiology.

● Thyroid hormones play an important role in the `color{violet}"regulation"` of the `color{violet}"basal metabolic rate"`.

● These hormones also support the process of `color{violet}"red blood cell formation"`.

● Thyroid hormones control the metabolism of `color{violet}"carbohydrates, proteins and fats"`.

● Maintenance of `color{violet}"water and electrolyte balance"` is also influenced by thyroid hormones.

● Thyroid gland also secretes a `color{violet}"protein hormone"` called `color{brown}"thyrocalcitonin (TCT)"` which regulates the `color{violet}"blood calcium levels"`.

● In humans, `color{violet}"four parathyroid glands"` are present on the `color{violet}"back side"` of the thyroid gland, one pair each in the `color{violet}"two lobes"` of the thyroid gland.

● The parathyroid glands secrete a `color{violet}"peptide hormone"` called `color{brown}"parathyroid hormone (PTH)"`.

● The secretion of PTH is regulated by the `color{violet}"circulating levels"` of `color{violet}"calcium ions"`.

● Parathyroid hormone (PTH) `color{violet}"increases"` the `Ca2^+` levels in the blood.

● PTH acts on `color{violet}"bones"` and stimulates the process of `color{brown}"bone resorption"` (`color{violet}"dissolution/ demineralisation"`).

● PTH also stimulates `color{violet}"reabsorption"` of `Ca2^+` by the `color{violet}"renal tubules"` and increases `Ca2^+` absorption from the `color{violet}"digested food"`.

● It is, thus, clear that PTH is a `color{violet}"hypercalcemic hormone"`, i.e., it increases the blood `Ca2^+` levels.

● Along with `color{violet}"TCT"`, it plays a significant role in `color{violet}"calcium balance"` in the body.