Virtual Medical Centre

Endocrine System

• Introduction to the endocrine system
• The pituitary gland
• The thyroid gland
• The parathyroid glands

Introduction to the endocrine system

Image courtesy of Blausen Medical Communications. Contact Andrew Walbank.

The role of the endocrine system is to maintain the body in balance through the release of hormones (chemical signals) directly into the bloodstream. Hormones transfer information and instructions from one set of cells to another. Many different hormones move through the bloodstream, but each type of hormone is designed to affect only certain cells.

A gland is a group of cells that produces and secretes chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body. The endocrine gland cells release a hormone into the blood stream for distribution throughout the entire body. These hormones act as chemical messengers and can alter the activity of many organs at once.

The parts of the endocrine system are grouped together because they release hormones into the blood without going through a duct (which is basically a tube) first. This is different to an exocrine gland, which releases what it creates through a tube to somewhere other than the blood.

Hormones can act on some specific cells because they themselves do not actually cause an effect. It is only through binding with a receptor (part of the cell specifically designed to recognize the hormone) like a key into a lock - that causes a chain reaction to occur, changing the activity of the cells. If a cell does not have a receptor for a hormone then there will be no effect. Also, there can be different receptors for the same hormone, and so the same hormone can have different effects on different cells.

The pituitary gland

Image courtesy of Blausen Medical Communications. Contact Andrew Walbank.

The pituitary gland is a small, oval gland lying at the base of the brain. It is divided into two sections - an anterior (meaning front) and posterior (meaning back) part because they are formed in different ways. The anterior pituitary is a collection of hormone-producing cells. The release of the hormones by these cells is controlled by a region of the brain called the hypothalamus

The posterior pituitary is made up of around 50,000 nerve endings. These nerves release their hormones straight into the blood.

Numerous hormones are released from the anterior pituitary. They are:

• Thyroid-stimulating hormone (TSH): This stimulates the thyroid gland.
• Adrenocorticotrophic hormone (ACTH): This stimulates the adrenal glands.
• Follicle-stimulating hormone (FSH): Promotes development of eggs within the ovaries and stimulates the secretion of oestroges (the primary female hormone). In men, FSH is important for sperm production in the testes.
• Luteinizing hormone (LH): Causes ovulation in women and prepares the uterus for pregnancy.
• Prolactin (PRL): Prolactin causes the development of breast tissue and the production of milk.
• Growth hormone (GH): Also called Somatotropin, it causes growth in almost all tissues in the body that are capable of growing. It promotes both an increase in cell size, and cell number.

Those hormones released from the posterior pituitary are:

• Anti-diuretic hormone (ADH): ADH causes the kidneys to keep more water in the body.
• Oxytocin: This causes contractions in the uterus of a pregnant woman and also causes the release of milk from the breast

The thyroid gland

Image courtesy of Blausen Medical Communications. Contact Andrew Walbank.

The thyroid (meaning 'shield-shaped') gland sits in the centre of the neck, a the front, below the Adam's apple. It is made of two lobes joined in the centre. At 15 to 20 grams it is one of the largest of the endocrine glands.

The thyroid secretes two major hormones called thyroxine (T₄) and triiodothyronine (T₃). They cause lots of things, but mostly they increase the rate of metabolism in the body. Metabolism is the amount of energy used by the body. An increase means more energy sources like fats and sugars are being broken down, and the body is using more energy to grow. The thyroid is controlled mainly by the release of Thyroid Stimulating Hormone (TSH) from the pituitary gland. The thyroid also secretes a hormone called calcitonin, important in keeping calcium levels in the body normal.

To create the thyroid hormones, the body needs a substance called iodine, which is found mainly in salt.

Low levels of thyroid hormone can result in feelings of tiredness, excessive sleep, loss of sex-drive, and smaller, less frequent periods in a woman.

Calcitonin is another thyroid hormone and this assists in the regulation of calcium concentration in body. Calcitonin lowers plasma calcium levels by inhibiting the cells which break down bone, and stimulating calcium excretion by the kidneys.

The parathyroid glands

The parathyroid glands are small, ovoid, and lie on the back of the thyroid gland. Most people have four parathyroid glands, two at the top, and two at the bottom.

There are two types of cell within the parathyroid gland. While calcitonin is released from the thyroid when calcium levels are too high, the parathyroids release their hormone when calcium levels are too low.