A chemical messenger molecule secreted (i.e., released) by endocrine cells within endocrine glands.

By definition, a hormone molecule is released into the bloodstream and travels throughout the body to find its target cells. Teget cells may be within the intravascular compartment (i.e., within blood vessels), but most hormones have target cells in tissues outside of the blood vessels.

Endocrine cell
Endocrine gland

Neurohormones are chemical messenger molecules that are released by neurons, but enter the bloodstream where they travel to distant target sites within the body. Therefore, neurohormones share characteristics with both neurotransmitters and hormones. Similar to neurotransmitters, neurohormones are released by neurons. Similar to hormones, neurohormones travel in the bloodstream.

Two well-known examples of neurohormones are oxytocin and the antidiuretic hormone (also referred to as vasopressin).

Hormone
Neurotransmitter

The action potential is a rapid and reversible reversal of the electrical potential difference across the plasma membrane of excitable cells such as neurons, muscle cells and some endocrine cells. In a neuronal action potential, the membrane potential rapidly changes from its resting level of approximately -70 mV to around +50 mV and, subsequently, rapidly returns to the resting level again. The neuronal action potential forms an important basis for information processing, propagation, and transmission. In muscle cells, the action potential precedes, and is necessary to bring about, muscle contraction. Some endocrine cells also exhibit action potentials, where the excitation leads to hormone secretion.

The action potential is also referred to as the electrical impulse or nervous impulse.

Graded potential

Neuronal Action Potential

Another name for the parafollicular cell type within the thyroid gland. C cells are situated outside of the thyroid follicles and are interspersed between the follicles. C cells are responsible for producing the hormone calcitonin.

Parafollicular cell

Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing adrenocorticotropic hormone (ACTH).

Mental retardation, dwarfism, and abnormal bone formation caused by thyroid hormone deficiency during fetal development.

The enzyme found in target tissues of the thyroid hormones that converts thyroxine (also known as tetraiodothyronine or T₄) to triiodothyronine (T₃).

Specifically, 5'-deiodinase converts T₄ to the active form of the thyroid hormone, T₃.

5-Deiodinase converts T₄ to the inactive form of the thyroid hormone, reverse T₃ (rT₃).

Endocrine cells are responsible for producing and releasing hormone molecules into the bloodstream. Endocrine cells are typically grouped together in organs referred to as endocrine glands.

Endocrine gland
Hormone

Endocrine glands contain clusters of endocrine cells, whose function is to is to release hormones into the bloodstream.

Endocrine glands are ductless glands in that their secretions do not enter a duct (as for example with salivary glands). Rather, endocrine glands release their secretions (i.e., messenger molecules) into the interstitial fluid in highly vascularized regions, where the molecules then enter the bloodstream. The chemical messenger molecule that is released into the bloodstream is referred to as a hormone.

Endocrine cell
Hormone

Goiter is an enlargement of the thyroid gland caused most commonly by iodine deficiency in the diet. Iodine deficiency leads to low levels of thyroid hormone production, and a reduction in thyroid hormone negative feedback on the hypothalamus and the anterior pituitary leads to a compensatory rise in the thyroid stimulating hormone (TSH). Enlargement of the thyroid gland is, therefore, caused by abnormally high circulating levels of TSH, which has a strong trophic effect on the thyroid gland.

Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing follicle stimulating hormone (FSH) and luteinizing hormone (LH).

A type of primary hypothyroidism, Hashimoto's disease is an autoimmune disorder, in which anti-thyroid antibodies destroy the ability of the thyroid gland to produce the thyroid hormones. Antibodies against thyroid peroxidase (TPO) and thyroglobulin (Tg) are often seen in this disorder.

Hashimoto's disease is also known as Hashimoto's thyroiditis.

Hypothyroidism
Primary hypothyroidism

Hyperthyroidism refers to a pathophysiological condition in which the thyroid gland produces and releases abnormally high levels of the thyroid hormones (T₃ and T₄).

The symptoms of hyperthyroidism may include high metabolic rate, weight loss, nervousness, excess heat production, tachycardia, and tremor.

Two main forms of hyperthyroidism exist: (1) Primary hyperthyroidism, and (2) Secondary hyperthyroidism.

In primary hyperthyroidism, the thyroid gland produces high levels of the thyroid hormones, either as a result of a secretory tumor of the thyroid gland, or under the control of thyroid stimulating immunoglobulins (such as in Graves' disease).

Secondary hypothyroidism is caused by high levels of thyroid stimulating hormone (TSH) produced by the anterior pituitary gland. TSH then stimulates the thyroid gland to produce excessive amounts of the thyroid hormones.

Hypothyroidism
Euthyroidism

Underactivity of the anterior pituitary gland characterized by reduced secretion of anterior pituitary hormones.

Hypothyroidism refers to a pathophysiological condition in which the thyroid gland does not produce sufficient amounts of the thyroid hormones (underactive thyroid), leading to abnormally low levels of the thyroid hormones (T₃ and T₄).

The symptoms of hypothyroidism may include reduced metabolic rate, chronic fatigue, weight gain, myxedema, and depression.

Three main forms of hypothyroidism exist: (1) Primary hypothyroidism, (2) Secondary hypothyroidism, and (3) Tertiary hypothyroidism.

In primary hypothyroidism, the thyroid gland itself is incapable of producing normal levels of the thyroid hormones.

Secondary hypothyroidism is caused by low levels of thyroid stimulating hormone (TSH). TSH, produced by the anterior pituitary gland, is required to stimulate the thyroid gland to produce the thyroid hormones.

Tertiary hypothyroidism is caused by low levels of thyrotropin releasing hormone (TRH). TRH, produced by the hypothalamus, is required to stimulate the anterior pituitary gland to produce TSH which, in turn, is required to stimulate the thyroid gland to produce the thyroid hormones.

Primary hypothyroidism
Secondary hypothyroidism
Tertiary hypothyroidism

Hyperthyroidism
Euthyroidism

Refers to the ability of the thyroid gland to accumulate iodide (I⁻) against a steep electrochemical gradient. While the iodide concentration in plasma and interstitial fluid is approximately 300 nL, iodide concentration in the cytoplasm of thyroid follicular cells, as well as the lumen of thyroid follicles can be many folds higher. The protein that enables iodide transport into the thyroid gland against an electrochemical gradient is the Na⁺/iodide symporter (NIS), which is located in the basolateral membrane of thyroid follicular cells. Within the lumen of thyroid follicles, iodide is incorporated into the tyrosine residues of thyroglobulin during thyroid hormone biosynthesis, hence, allowing very high iodide concentrations in the colloid.

Cells within the thyroid gland. Parafollicular cells are situated outside of the thyroid follicles and are interspersed between the follicles. Parafollicular cells are responsible for producing the hormone calcitonin. Parafollicular cells are also referred to as C cells.

Plasma is the fluid portion of whole blood, which makes up about 40% to 60% of the total volume of whole blood. Plasma has a light yellow color and is generally obtained by separating the fluid portion from the blood formed elements through sedimentation or centrifugation. Plasma contains mostly water and, in addition, minerals, nutrients, proteins, hormones, and gases (oxygen and carbon dioxide). Unlike serum, in which fibrinogen and other clotting factors have been removed by coagulation, fibrinogen and other clotting factors remain present in plasma.

Plasma is one the main fluid compartments of the human body, making up nearly 10% of the total volume of body fluids. Plasma makes up the intravascular fluid compartment; itself a subcompartment of the extracellular fluid compartment.

Primary hypothyroidism is characterized by abnormally low levels of thyroid hormone (T₃ and T₄) production, where the defect is at the level of the thyroid gland itself.

In primary hypothyroidism, the circulating levels of the thyroid hormones are low, however, the levels of the thyroid stimulating hormone (TSH) are high due a lack of thyroid hormone negative feedback on the anterior pituitary.

Common causes of primary hypothyroidism include iodine deficiency (which leads to goiter), and Hashimoto's disease, in which autoimmune antibodies destroy the ability of the thyroid gland to produce the thyroid hormones.

Hypothyroidism
Secondary hypothyroidism
Tertiary hypothyroidism

Peptide hormone produced by the anterior pituitary gland. Prolactin is best known for its action in stimulating the mammary glands to produce milk (lactation).

Prolactin is known to be involved in many other physiological processes including enlargement of the mammary glands in preparation for milk production, sexual gratification, metabolism, regulation of the immune system, and others.

Prolactin (Wikipedia)

Secondary hypothyroidism is characterized by abnormally low levels of thyroid hormone (T₃ and T₄) production, where the defect is at the level of the anterior pituitary gland.

In secondary hypothyroidism, the anterior pituitary gland is unable to produce sufficient levels of the thyroid stimulating hormone (TSH) which, in turn, leads to insufficient stimulation of the thyroid gland to produce the thyroid hormones (T₃ and T₄).

Thus, in secondary hypothyroidism, the circulating levels of both TSH and the thyroid hormones (T₃, and T₄) are abnormally low

Hypothyroidism
Primary hypothyroidism
Tertiary hypothyroidism

Secretion refers to cellular release of substances (ions and small and large molecules) to the external environment of the cell. Secretion may be accomplished by exocytosis (fusion of transport vesicles with the plasma membrane and release of vesicle contents to the external environment), by transport of molecules across the plasma membrane (via the activity of transport proteins such as pumps, transporters, and channels), or by simple diffusion of fat-soluble molecules through the plasma membrane out of the cell.

For example, endocrine cells secrete hormone molecules that then enter the bloodstream. Neurons release (i.e., secrete) neurotransmitter molecules into the synaptic cleft. Some neurons secrete neurohormones; which similar to hormones, travel in the bloodstream to reach distant target cells. Epithelial cells secrete molecules in luminal spaces, such as digestive enzymes secreted into the digestive tract by various cell types.

Excretion

Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing growth hormone (GH).

Tertiary hypothyroidism is characterized by abnormally low levels of thyroid hormone (T₃ and T₄) production, where the defect is at the level of the hypothalamus.

In tertiary hypothyroidism, the hypothalamus is unable to produce sufficient levels of thyrotropin release hormone (TRH). Low TRH levels lead to low production of the thyroid stimulating hormone (TSH) by the anterior pituitary gland which, in turn, leads to insufficient stimulation of the thyroid gland to produce the thyroid hormones (T₃ and T₄).

Thus, in tertiary hypothyroidism, the circulating levels of TRH, TSH, T₃, and T₄ are all abnormally low.

Hypothyroidism
Primary hypothyroidism
Secondary hypothyroidism

Tg

The major glycoprotein found within the colloid of thyroid follicles. The thyroid hormones (T₃ and T₄) are synthesized on the backbone of thyroglobulin.

Thyroid follicular cells are epithelial cells that form the lining of thyroid follicles. Each thyroid follicle is surrounded by a single layer of follicular cells. Follicular cells play the key role in thyroid hormone synthesis and release.

Thyroid follicular cells are also referred to as thyrocytes.

Endocrine cell of the anterior pituitary gland responsible for synthesizing and releasing thyroid stimulating hormone (TSH).

The part of the embryonic gonad that develops into the male reproductive ducts (epididymis, vas deferens, and seminal vesicles). In the female, this structure disappears because of the absence of anti-Müllerian hormone secreted by the Sertoli cells.