Endocrine system

The endocrine system regulate development, growth, energy metabolism, reproduction, and many different behaviors. It uses a combination of glands and hormones to send signals to the nervous system and control functions in the body.

Hormones are the chemicals the endocrine system uses for signalling. Several different glands exist in the body which release hormones. These hormones travel through bodily fluid, usually blood, to the target cells.

Function
Three types of hormones may be released: lipid-derived hormones such as steroids, amino-acid hormones (e.g., epinephrine and norepinephrine), and peptide hormones (e.g., insulin and antidiuretic hormone. Steroid hormones permeate tissues and can directly enter the nucleus to trigger DNA to start protein production. Protein hormones do not share this mechanism because they are unable to enter the nucleus due to their membranes being incompatible with the nuclear membrane. Instead, protein hormones attach to messenger receptor cell on the outside of the target tissues' cells. Based on the hormone that attaches, a different cascade of cellular processes will occur ending in tissue response (which could be increased enzymatic activity, cellular secretion, and ion channel opening or closing).

Pituitary
The pituitary gland is a small bulb situated at the base of the brain and both releases hormones and controls other glands which release hormones. It is under direct control of the hypothalamus, a section of the brain right above the pituitary. The hypothalamus receives input about hormonal and homeostatic states in the body, processes that information, then relays instructions to the pituitary gland to respond with certain hormones. For example, osmoreceptors, water-level sensing cells, in the hypothalamus detect when you are dehydrated. The hypothalamus tells the pituitary to release antidiuretic hormone, which notifies the bladder to retain water. Water levels in the body will increase and once at an adequate level the osmoreceptors stop their signaling to have ADH released.

Pituitary dysfunction can lead to several disorders, including:
 * Acromegaly - an overproduction of growth hormone which can cause gigantism in children and oversized limbs in adults.
 * Cushing's Disease - usually caused by a tumor on the pituitary, it is an overproduction of ACTH, the hormone that stimulates the production of cortisol. It leads to consistent high levels of cortisol in the bloodstream leading to symptoms such as increased fat around the neck, memory and cognitive dysfunction, and osteoporosis.
 * Hypopituitarism - when the pituitary doesn’t produce enough hormones; can cause anemia, decreased appetite, weight changes, joint stiffness, and developmental problems (in children).

Hypothalamus
The hypothalamus maintains homeostasis in the body. It receives input from sources such as the vagus nerve (which can relay information about blood pressure and stomach fullness), the optic nerve (which relays levels of light), receptors in the body and skin (which relay temperature), and even has its own receptors which inform about the amount of molecules in the blood (which relays if there is an unbalanced ratio of fluid to molecules).

When the hypothalamus is not functioning properly, many systems become dysregulated. This dysfunction can be caused by a number of different triggers such as an eating disorder, a tumor, brain injury or genetic mutations. A malfunctioning hypothalamus alters motivations to drink, eat and copulate which can lead to malnutrition or overeating, dehydration or overhydration and decreased or increased sexual behavior. Sleep patterns, activity levels, and menstruation may also change.

Thyroid
The thyroid is a gland located in the throat. It is necessary in proper brain functioning and health, and is important to the development of children and fetus’s brains. The gland also plays a role in controlling the breakdown and use of proteins, carbohydrates and fats. The thyroid has cells that absorb iodine. The cells use iodine to produce T3 and T4, thyroid hormones that can moderate the body’s temperature, and vitals (e.g. blood pressure and heart rate), and the breakdown of various biomolecules such as fat.

Dysfunction of this gland can mean an overproduction or underproduction of its hormones. Malfunction can lead symptoms such as to hair loss, muscle fatigue and weakness, sensitivity to environmental temperature changes.

Relationship to ME/CFS
Hypothalamic-pituitary-adrenal axis (HPA) is the collaborative functioning between the hypothalamus and the pituitary gland. These tissues work together to regulate hormone production and release, and also moderate bodily functions such as temperature regulation, fluid balance, appetite and thirst detection. Hypofunction of the HPA has been identified as a problem experienced in ME/CFS patients. Lower cortisol levels were identified in ME/CFS patients than in healthy controls, and has been shown to be connected to decreased sleep. Exogenous cortisol administration was tested as a possible treatment however these results should be replicated due to the group’s use of solely the Fukuda Assessment of ME/CFS (which may allow for error in diagnosis of the condition). Furthermore, several journals suggested CBT as a feasible treatment option to increase cortisol levels, however due to the extensive literature questioning and undermining the utility of CBT in ME/CFS in overall treatment, other options should be explored in this research.