ANTIDIURETIC HORMONE ( ADH ) This is also referred to as vasopressin and is released in response to the hydration status of the patient , specifically in response to changes in the volume of extracellular fluid , e . g . blood plasma . ADH primarily acts on the collecting ducts in the renal nephrons and causes changes to the permeability of the ducts to counteract dehydration and over-hydration . In cases of dehydration , more ADH is released , causing more water to be reabsorbed from the urine and returning it to the plasma , thereby decreasing urine production . Conversely , in cases of over-hydration , ADH is not secreted , and water is not reabsorbed from urine and is excreted from the body . Diabetes insipidus is a disorder of the pituitary gland , whereby it stops secreting ADH , which means water reabsorption in response to blood volume does not occur ; these patients are polyuric and polydipsic and at risk of severe dehydration if they are unable to meet the increased urine losses with water intake .

Thyroid gland

The thyroid gland consists of a pair of lobes located either side of the trachea , usually at the level of the first or second tracheal rings , and is an important endocrine gland for metabolic control . The thyroid gland consists of two types of cells : follicular cells and parafollicular cells .

FOLLICULAR CELLS Thyroxine ( T4 ) and triiodothyronine ( T3 ) are secreted from the follicular cells ( also referred to as thyroid epithelial cells ). The T4 and T3 thyroid hormones are the primary influences on the body ’ s basal metabolic rate ( Petroff & Greco , 2020 ) so any disturbances to the levels of these hormones will affect this ; many of the clinical signs observed with thyroid disorders are due to this . The thyroid gland needs iodine to be able to secrete thyroid hormones .

PARAFOLLICULAR CELLS ( OR C CELLS ) Calcitonin is secreted by the parafollicular cells ( Kipperman & Rogers , 2012 ) and has an important role in the regulation of calcium levels in the body . Calcitonin secretion is controlled by the levels of calcium detected in the blood – if calcium levels are high , calcitonin is released to stimulate the deposition of calcium into the bones , thereby lowering the levels of calcium in the blood . Calcitonin works in conjunction with parathyroid hormone ( discussed below ) to control calcium levels .

Parathyroid glands

The parathyroids are bilateral pairs of glands ( so four in total ) lying either side of the trachea near the thyroid gland . Only one hormone is synthesised and secreted by these endocrine glands :

PARATHYROID HORMONE ( PTH ) The main role of PTH is to regulate calcium levels in the blood and , as discussed above , works in conjunction with calcitonin and counteracts its effect . When low levels of calcium in the blood ( hypocalcaemia ) are detected , PTH is secreted by the parathyroids in response to this . PTH raises the levels of calcium in the blood by three main actions :

• Increases absorption of calcium in the intestines

• Increases retention of calcium in the kidneys

• Increases osteoclast activity which , in turn , increases release of calcium from bone stores

Pancreas

( Kipperman & Rogers , 2012 )

The pancreas is classified as a mixed gland as it has both endocrine and exocrine functions ( involving the release of digestive enzymes into the duodenum via the pancreatic duct ). The endocrine part of the pancreas is found in the islets of Langerhans and these islets consist of three types of cells – alpha , beta and delta cells – each of which are responsible for secreting a different hormone . The endocrine pancreas is important for the control of glucose and insulin levels in the blood , and deficiencies / disturbances in these hormones will result in diabetes mellitus .

ALPHA CELLS These cells are responsible for the secretion of glucagon in response to a decrease in blood glucose ( BG ). The release of glucagon , in conjunction with adrenaline and corticosteroids , causes glycogen ( stored in the liver ) to be converted into glucose by the process of glycogenolysis while also increasing the rate of new glucose production from amino acids by the process of gluconeogenesis ( Petroff & Greco , 2020 ) – the final result is an increase in BG levels . Glucagon also inhibits the action of insulin in the body .

BETA CELLS Insulin is secreted from the beta cells in the pancreas in response to increasing BG levels , normally immediately after a meal . Insulin works by increasing the uptake of glucose into muscle cells to be converted into energy ; it also converts glucose into glycogen ( by the process of glycogenesis ) to be stored in the liver ready for release when BG levels fall and by converting any leftover glucose into fat . Insulin is the only hormone in the body that acts to lower BG levels .

DELTA CELLS The third hormone , released from the delta cells in the endocrine pancreas , is somatostatin . This hormone works by balancing the effects of glucagon and insulin to control BG fluctuations .

Adrenal glands

The adrenal glands are paired structures located close to the cranial aspect of each kidney . Each gland consists of an outer cortex and inner medulla ; both layers act independently of each other and are controlled by different mechanisms .

56 Veterinary Nursing Journal