Biology

Parathyroid glands  are small   endocrine   glands  in the neck of humans and other  tetrapods . Humans usually have four parathyroid glands, located on the back of the  thyroid gland  in variable locations.   The parathyroid gland produces and secretes  parathyroid hormone  in response to a low blood calcium, which plays a key role in regulating the amount of calcium in the blood and within the bones .  Parathyroid hormone (also known as parathormone) is a small   protein   that takes part in the control of calcium and phosphate   homeostasis , as well as bone physiology. Parathyroid hormone has effects antagonistic to those of   calcitonin . Calcium . PTH increases blood calcium levels by directly stimulating osteoblasts and thereby indirectly stimulating  osteoclasts  (through RANK/RANKL mechanism) to break down bone and release calcium. PTH increases gastrointestinal calcium absorption by activating  vitamin D , and promotes calcium conservation (reabsorption) by the  ki

The thyroid gland is a butterfly-shaped organ that is located at the front of the neck. The thyroid gland produces three hormones: Triiodothyronine, also known as T3 Tetraiodothyronine, also called thyroxine or T4 Calcitonin Here, the highlighted portion is the Thyroid Gland. Hormonal output from the thyroid is regulated by Thyroid Stimulating Hormone  (TSH) secreted from the Anterior Pituitary  gland, which itself is regulated by Thyrotrophin - Releasing Hormone  (TRH) produced by the hypothalamus. The thyroid may be affected by several  diseases .   Hyperthyroidism  occurs when the gland produces excessive amounts of thyroid hormones, the most common cause being  Graves' disease , an   autoimmune disorder . In contrast,  hypothyroidism  is a state of insufficient thyroid hormone production. Worldwide, the most common cause is  iodine deficiency .

The Pineal gland is a small, pea-shaped gland in the brain. It secretes Melatonin hormone ........ Melatonin is a hormone from the pineal gland that helps regulate  biological rhythms  such as sleep and wake cycles. In other words, it's melatonin that directs your internal clock - the one that allows you to wake up about the same time every morning without an alarm clock.