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This essay will describe the structure, function, and interrelationship of one of the body’s major systems, the endocrine system, and illustrate its malfunction and associated causes, symptoms, and treatment.
A sporadic network across the body, the endocrine system, consisting of glands and organs with no physical connections, produce and secrete chemical messages called hormones. The term hormone, derived from the Greek word ‘hormao’, meaning ‘I excite’, refers to each hormone exiting or stimulating a particular part of the body or target gland. The endocrine glands comprise groups of secretory cells surrounded by an extensive network of capillaries that transport and diffuse hormones in the bloodstream to specifically targeted tissues and organs to regulate and control a wide range of bodily functions, such as respiration, metabolism, reproduction, sensory perception, movement, sexual development, and growth.
The major hormone-producing glands are the pituitary gland, situated in a hollow behind the bridge of the nose, the sella turcica, and attached to the base of the brain, which produces the hormones that cause growth and, considered the ‘master control gland’, controls other glands, including the adrenals, producing cortisol, thyroid gland, producing thyroxin, and the sex glands, ovaries, and testes. The pineal gland, producing melatonin, affects sleep and regulates the body’s circadian clock, however, this gland is not fully understood. The hypothalamus, situated on the underside of the brain, is responsible for homeostasis, regulating bodily temperature, moods, hunger, thirst, and sex drive, and dictating the discharge of hormones from other glands. The hypothalamus produces an anti-diuretic hormone to aid water reabsorption in the kidney and oxytocin, which is vital to aid breastfeeding. The parathyroid gland, producing the parathyroid hormone, controls the amount of calcium in the blood, which is vital for nerves and muscles to remain effective and bones to remain strong. The thyroid, butterfly-shaped and located in the neck, produces thyroxin, thyrocalcitonin, and triiodothyronine, hormones responsible for metabolism and heart rate digestive functions, brain function and development, muscle control, and maintenance of bones. The pancreas, found in the abdomen, controls blood sugar levels by producing insulin and glucagon. Producing hormones essential for a healthy life, the adrenal glands, triangularly shaped and found situated at the top of both kidneys, produce androgens and estrogens that control sex, aldosterone to balance the salt in the blood, and cortisol to balance sugar, proteins, carbohydrates, fats, suppress inflammation and regulates blood pressure. Catecholamines or adrenaline-type hormones such as epinephrine, also known as adrenaline, and norepinephrine, otherwise known as noradrenaline, produced by the adrenal medulla, are essential to the body’s fight or flight response. Present solely in the female body, the ovaries secrete the female sex hormones, estrogen, testosterone, and progesterone. Present only in the male body, the testes produce the male sex hormone, testosterone, and are responsible for sperm production. The kidneys, an organ with a secondary endocrine function, produce erythropoietin, a hormone responsible for the regulation of red blood cell production. Hypoxia, which can result from, for example, low blood flow, low blood volume, anemia, or lung disease, is the major stimulus for increased erythropoietin production to raise oxygen levels and restore homeostasis.
For a hormone to affect a target cell, one of two main mechanisms must take place. In the first, non-steroid action, amino acid derivative hormones such as melatonin, or peptide hormones, for example, oxytocin, cannot cross the target cell membrane freely and so bind to specific receptors on the cell surface where they cause an intercellular signaling cascade. The second type, steroid action, see lipophilic lipid and steroid hormones, pass freely through the membrane and bind to receptors inside the cell or move into the nucleus. Both hormone actions set in motion a negative feedback mechanism that persists until homeostasis is restored.
Negation of normal or optimal hormonal range is established with negative feedback mechanisms that prevent sudden, serious changes in homeostasis. One physiological variable, core temperature, can be used to exemplify the three basic components of a system control maintaining homeostasis. The detector, nerve endings in the skin, detect a temperature decrease or increase from the normal, around thirty-seven degrees Celsius, then relay this information to the system control, the hypothalamus gland, leading to the activation of the mechanisms that control temperature, the effector. Examples of this mechanism or effector are reducing blood flow by narrowing blood vessels to prevent heat loss or, conversely, behavioral changes, such as jumping up and down to warm the body. Once the normal temperature has been re-established and the stimuli ceased, signals to the hypothalamus stop, and homeostasis is achieved.
A rare and lifelong condition, congenital hypothyroidism, is a malfunction of the endocrine system resulting from an under-developed, absent, or malfunctioning thyroid gland whereby the body cannot produce thyroxine and triiodothyronine, which are essential for brain development in infancy and typical growth in childhood and adolescence. A homeostatic level of thyroxin is also essential for the rise in sex hormones responsible for puberty. Congenital hypothyroidism, tested for in all newborns at five days old, sometimes called a ‘heal prick’ test or ‘Guthrie’ test, takes a small blood sample from the heel and is sent for analysis in a laboratory. This test also checks for eight other serious conditions: phenylketonuria, sickle cell disorders, cystic fibrosis, medium-chain acyl-CoA dehydrogenase deficiency, maple syrup urine disease, isovaleric acidemia, glutaric aciduria type one and homocystinuria. According to Great Ormond Street Hospital, “in around one in every 3500 newborns have congenital hypothyroidism”, which is less common in boys than girls, however, science is yet to discover why (GOSH, 2015). After the heel prick test, the child will undergo a nuclear medicine thyroid scan, using a small amount of radioactive iodine to obtain a picture of the thyroid to determine its function and confirm a diagnosis. Whilst most cases of congenital hypothyroidism are diagnosed very early and before any symptoms are apparent, they may present as jaundice, feeding difficulties, constipation, lethargy, cold extremities, low muscle tone, and poor growth. If diagnosis and treatment are not carried out almost immediately after birth, the child may display developmental delays, learning difficulties, and clumsiness.
Managed by a pediatric endocrinologist, treatment with the drug levothyroxine, a synthetic thyroid hormone (T4), is prescribed, with dosage calculated based on the weight of the child, and regularly assessed with blood tests to measure thyroid-stimulating hormone. The patient is prescribed levothyroxine (T4), a manufactured thyroxine substitute, as it is needed for the body to produce triiodothyronine (T3).
Summing up, the endocrine system, with its homeostatic mechanisms of a receptor, control system, and effector, maintains the stability of physiological functions such as metabolism, sensory perception, growth, respiration, body temperature, and sexual development. These homeostatic mechanisms ensure physiological equilibrium, however, if malfunctioning, as with congenital hypothyroidism, medical intervention is required.
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