Endocrine System Animations

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Enzyme Amplification

Hormones are very potent chemicals even at very low concentrations due to enzyme
amplification. Even the slightest amount of hormone acting on the cell surface can initiate a powerful cascading activating force for the entire cell. View the animation below

Positive and Negative Feedback

The secretion of TSH, FSH, LH and ACTH from pituitary decreases when blood levels of their target gland hormones rise. This is called negative feedback. The other type of feed back control is positive feed back. View the animation below to understand positive and negative feedback mechanisms.

Mechanism of Hormone Action:
Lipid-soluble Hormones

Lipid-soluble hormones such as steroid hormones diffuse through lipid bilayer of the plasma membrane. They bind to intracellular hormone receptors. The activated
receptor-hormone complex influences gene expression.

Mechanism of Hormone Action:
Water-soluble Hormones

Unlike lipid-soluble hormones, water-soluble hormones like protein hormones, peptide
hormones and catecholamines cannot diffuse through lipid bilayer of the plasma membrane. They bind to receptors on the target cell membrane. This causes the production of a second messenger e.g. cAMP (cyclicAdenosineMonoPhosphate) inside the cell. The cAMP then causes subsequent intracellular effects of the hormone by activating a cascade of enzymes.

Mechanism of Thyroxine Action

Thyroid hormones are derived from the amino acid tyrosine and they have iodine incorporated into their structures. There are two thyroid hormones, which have either 3 or 4 atoms of iodine per molecule; they are known respectively as T3 (tri-iodothyronine) and T4 (thyroxine). Both are synthesized within the thyroid follicles and secreted into the bloodstream when the cells are stimulated by TSH to extrude them. When the appropriate level of T3/T4 is attained in the blood, thyroid hormones switch off TSH production. This is an example of a negative feedback system.
T3, the form of thyroid hormone which contains only 3 atoms of iodine per molecule, is now considered to be the physiologically active hormone, and T4 to be a precursor of T3, which can be converted to T3 by specific enzymes within the target cells. Since T4 circulates at a concentration about a hundred-fold higher than that of T3, it can therefore be considered to be a storage form of the active hormone.

Osteoporosis

Osteoporosis literally means "porous bones. PTH increases the activity of osteoclasts. This results in resorption of calcium phosphate ions from bone into the blood. Hypersecretion of PTH results in osteoporosis.