Endocrinology (from Greek ἔνδονendon, "within"; κρίνωkrīnō, "to separate"; and -λογία-logia) is a branch of biology and medicine dealing with the endocrine system, its diseases, and its specific secretions known as hormones. It is also concerned with the integration of developmental events proliferation, growth, and differentiation, and the psychological or behavioral activities of metabolism, growth and development, tissue function, sleep, digestion, respiration, excretion, mood, stress, lactation,movement, reproduction, and sensory perception caused by hormones. Specializations include behavioral endocrinology and comparative endocrinology.

The endocrine system consists of several glands, all in different parts of the body, that secrete hormones directly into the blood rather than into a duct system. Hormones have many different functions and modes of action; one hormone may have several effects on different target organs, and, conversely, one target organ may be affected by more than one hormone.


The endocrine system

 Endocrinology is the study of the endocrine system in the human body. This is a system of glands which secrete hormones. Hormones are chemicals which affect the actions of different organ systems in the body. Examples include thyroid hormone, growth hormone, and insulin. The endocrine system involves a number of feedback mechanisms, so that often one hormone (such as thyroid stimulating hormone) will control the action or release of another secondary hormone (such as thyroid hormone). If there is too much of the secondary hormone, it may provide negative feedback to the primary hormone, maintaining homeostasis.

In the original 1902 definition by Bayliss and Starling (see below), they specified that, to be classified as a hormone, a chemical must be produced by an organ, be released (in small amounts) into the blood, and be transported by the blood to a distant organ to exert its specific function. This definition holds for most "classical" hormones, but there are also paracrine mechanisms (chemical communication between cells within a tissue or organ), autocrine signals (a chemical that acts on the same cell), and intracrine signals (a chemical that acts within the same cell). A neuroendocrine signal is a "classical" hormone that is released into the blood by a neurosecretory neuron (see article onneuroendocrinology).



Griffin and Ojeda identify three different classes of hormone based on their chemical composition:


  • Amines

    Amines, such as norepinephrine, epinephrine, and dopamine (catecholamines), are derived from single amino acids, in this case tyrosine. Thyroid hormones such as 3,5,3’-triiodothyronine (T3) and 3,5,3’,5’-tetraiodothyronine (thyroxine, T4) make up a subset of this class because they derive from the combination of two iodinated tyrosine amino acid residues.
  • Peptide and protein

    Peptide hormones and protein hormones consist of three (in the case of thyrotropin-releasing hormone) to more than 200 (in the case of follicle-stimulating hormone) amino acid residues and can have a molecular mass as large as 30,000 grams per mole. All hormones secreted by the pituitary gland are peptide hormones, as are leptin from adipocytes, ghrelin from the stomach, and insulin from the pancreas.
  • Steroid

    Steroid hormones are converted from their parent compound, cholesterol. Mammalian steroid hormones can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids,androgens, estrogens, and progestogens. Some forms of vitamin D, such as calcitriol, are steroid-like and bind to homologous receptors, but lack the characteristic fused ring structure of true steroids.


Broadly speaking, endocrine disorders may be subdivided into three groups: 

  • Endocrine gland hyposecretion (leading to hormone deficiency)
  • Endocrine gland hypersecretion (leading to hormone excess)
  • Tumours (benign or malignant) of endocrine glands

Endocrine disorders are often quite complex, involving a mixed picture of hyposecretion and hypersecretion because of the feedback mechanisms involved in the endocrine system. For example, most forms of hyperthyroidism are associated with an excess of thyroid hormone and a low level of thyroid stimulating hormone.


 Glucose homeostasis disorders

  • Diabetes mellitus
    • Type 1 Diabetes
    • Type 2 Diabetes
    • Gestational Diabetes
    • Mature Onset Diabetes of the Young
  • Hypoglycemia
    • Idiopathic hypoglycemia
    • Insulinoma
    • Glucagonoma

Thyroid disorders

  • Goiter
  • Hyperthyroidism
    • Graves-Basedow disease
    • Toxic multinodular goitre
  • Hypothyroidism
  • Thyroiditis
    • Hashimoto's thyroiditis
  • Thyroid cancer
  • Thyroid hormone resistance

Calcium homeostasis disorders and Metabolic bone disease

  • Parathyroid gland disorders
    • Primary hyperparathyroidism
    • Secondary hyperparathyroidism
    • Tertiary hyperparathyroidism
    • Hypoparathyroidism
      • Pseudohypoparathyroidism
  • Osteoporosis
  • Osteitis deformans (Paget's disease of bone)
  • Rickets and osteomalacia

Pituitary gland disorders

Posterior pituitary

  • Diabetes insipidus

Anterior pituitary

  • Hypopituitarism (or Panhypopituitarism)
  • Pituitary tumors
    • Pituitary adenomas
    • Prolactinoma (or Hyperprolactinemia)
    • Acromegaly, gigantism
    • Cushing's disease

Sex hormone disorders

  • Disorders of sex development or intersex disorders
    • Hermaphroditism
    • Gonadal dysgenesis
    • Androgen insensitivity syndromes
  • Hypogonadism (Gonadotropin deficiency)
    • Inherited (genetic and chromosomal) disorders
      • Kallmann syndrome
      • Klinefelter syndrome
      • Turner syndrome
    • Acquired disorders
      • Ovarian failure (also known as Premature Menopause)
      • Testicular failure
  • Disorders of Puberty
    • Delayed puberty
    • Precocious puberty
  • Menstrual function or fertility disorders
    • Amenorrhea
    • Polycystic ovary syndrome

Tumours of the endocrine glands not mentioned elsewhere

  • Multiple endocrine neoplasia
    • MEN type 1
    • MEN type 2a
    • MEN type 2b
  • Carcinoid syndrome
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