Introduction to Endocrine

Question 1

What do I need to know about the endocrine system?

Answer 1

By the end of the lessons, students will be able to:
* Describe the basic structure and function of the endocrine system
* Describe pathophysiological processes of common thyroid and parathyroid gland disorders, and how they contribute to the presentation of disease
* Describe the signs and symptoms, aetiology, risk factors, pathophysiology, investigations, conventional medical treatments, complications and impact on wellbeing of common thyroid and parathyroid gland disorders.

Question 2

What is the structure and function of the endocrine system?

Answer 2

• Hormones are molecules (such as steroids, amino acid derivatives, polypeptides or proteins)
• They interact with receptors on their target cells, triggering biochemical reactions that modify the activity of the cell.
• Various glands located throughout the body produce hormones that regulate many functions, including growth and development, metabolism, the electrolyte balance and reproduction

Question 3

What is the THE HYPOTHALAMUS-PITUITARY COMPLEX?

Answer 3

The hypothalamus is located in the brain
* Produces hormones that act on the pituitary gland
* Hypothalamic hormones stimulate or inhibit the release of pituitary hormones

The pituitary gland is located in the brain, directly below the hypothalamus
* Some pituitary hormones act on other glands located in various parts of the body (such as adrenal glands, thyroid, testes and ovaries), which in turn release their own hormones
* Other pituitary hormones directly affect their target organs.

Target cells
* Stimulation of receptors on the target cell triggers biochemical reactions that modify the activity of the cell
* Negative feedback mechanisms control endocrine system activity - go/stop
* The hormones released from the target gland feedback to the pituitary and/or hypothalamus, repressing further hormone release and thereby downregulating the system

Question 4

What hormones are released by the hypothalamus?

Answer 4

In the hypothalamus
* Corticotropin-releasing hormone (CRH) = Stimulates the pituitary to release adrenocorticotropic hormone (ACTH)
* Gonadotropin-releasing hormone (GnRH) = Stimulates the pituitary to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
* Thyrotropin-releasing hormone (TRH) = Stimulates release of thyroid-stimulating hormone (TSH) from the pituitary
* Growth hormone-releasing hormone (GHRH) = Stimulates release of growth hormone (GH) from the pituitary
* Somatostatin = Inhibits release of GH from the pituitary
* Dopamine = Inhibits release of prolactin from the pituitary

Question 5

What hormones are released by the anterior pituatary gland?

Answer 5

• Adrenocorticotropic hormone (ACTH) = Stimulates release of hormones from the adrenal cortex
• Luteinizing hormone (LH) = Stimulates production of reproductive hormones in the ovaries and testes
• Follicle stimulating hormone (FSH) = Stimulates follicle development in the ovaries and sperm production in the testes
• Thyroid stimulating hormone (TSH) = Stimulates release of thyroid hormone
• Growth Hormone (GH) = Promotes growth and development
• Prolactin = Promotes lactation

Question 6

What hormones are released by other glands?

Answer 6

Posterior pituitary gland
* Vasopressin (ADH) = Produced by the hypothalamus but stored and released by the posterior pituitary. Helps to maintain water and electrolyte balance
* Oxytocin = Promotes uterine contraction during labour and activates milk ejection during breastfeeding

Pineal gland
* Melatonin = Helps control circadian cycle of sleep and wakefulness

Testes
* Testosterone = Stimulates reproductive system development, sperm production, and anabolism (growth)

Ovaries
* Oestrogen (produced by the follicle)
= Stimulates reproductive system development
* Progesterone (produced by the corpus luteum) = Prepares uterus for pregnancy and mammary glands for lactation

Thyroid gland
* Thyroxine (T4) and triiodothyronine (T3) = Controls metabolic processes in all cells
* Calcitonin = Helps control calcium metabolism (lowers blood calcium levels)

Parathyroid gland
* Parathyroid hormone (PTH) = Helps control calcium metabolism (increases blood calcium levels)

Pancreas
* Insulin = Helps control carbohydrate metabolism (lowers blood sugar levels)
* Glucagon = Helps control carbohydrate metabolism (increases blood sugar levels)

Question 7

What is normal hormone function influenced by?

Answer 7

The normal functioning of hormones in the body is influenced by:
* Nutrition and absorption of nutrients necessary for hormone production
* Transportation of hormones by blood, lymph, and digestive system
* Sensitivity of receptors that hormone is targeting
* Auto-immune targeting of a particular component in the hormone system, resulting in under or over production of hormones
* Tumour growth of hormone- secreting cells can mean too much hormone is released - the tumour acts independently
* Destruction of hormone-secreting cells by an adjacent tumour or other pathological
process, can lead to too little hormone being released

Question 8

What does the thyroid gland do?

Answer 8

One of the most common conditions seen in practice is underactive thyroid

Synthesises thyroid hormones
* Iodine is bound to thyroglobulin by the enzyme thyroid peroxidase to form T4 and T3, which are then are released into the peripheral
circulation from the thyroid gland
* T3 can also be formed by conversion of T4 to T3
* The majority of thyroid hormones in the circulation are inactive (bound to binding globulins)
* Free T3 is more biologically active than free T4s, so needs careful regulation

Regulates thyroid hormones
* TRH from the hypothalamus stimulates the release of TSH from the pituitary
* TSH binds to receptors on the thyroid gland and promotes production and release of T4 and T3
* Negative feed-back of T4 and T3 on the hypothalamus and pituitary regulates the HPT system
* Elevated levels of thyroid hormones (in hyperthyroid conditions) decreases TRH and TSH
* Insufficient levels of thyroid hormones results in an increase in TRH and TSH
levels

Hyperthryroid conditions - see a higher level of hormones but lower levels of TSH. This tells you the pituatary is trying to regulate things
Hypothyroid conditions - see a lower level of hormones produced by thyroid gland, you will see an elevated TSH but lower levels of T4. Pituatary is trying to provide extra stimulation
In early stages the pituitary can manage but the compensation can’t work for long