Introduction to Endocrinology

This set of vocabulary flashcards covers the fundamental definitions, classifications, and regulatory mechanisms of the endocrine system based on lectures 31 and 32.

Hormone

An extracellular signaling molecule that is released into the blood and acts at its receptors in distal tissues in order to elicit a physiological response.

Hydrophilic hormones

Hormones characterized by high water solubility and low lipid solubility.

Lipophilic hormones

Hormones characterized by high lipid solubility and low water solubility.

Amines

Amino acid derivatives like Dopamine, Norepinephrine, and Epinephrine (from tyrosine) or Melatonin (from tryptophan).

Peptide hormones

The most common class of hormones, consisting of short to medium length chains of $3$ to $200$ amino acids.

Steroid hormones

Lipophilic hormones all derived from cholesterol, including estrogens, androgens, progestins, glucocorticoids, and mineralocorticoids.

Thyroid hormones

Lipophilic hormones derived from $2$ tyrosines and added iodine, which include Thyroxine ($T_4$) and Triiodothyronine ($T_3$).

Steroidogenesis

The biochemical process of making steroids.

Negative feedback control

A regulatory mechanism driven either by direct sensing of changes in a controlled variable or by the endocrine axis itself.

Neuroendocrine reflexes

A sudden increase in hormone secretion in response to a specific stimulus, such as the release of epinephrine from the adrenal medulla.

Circadian rhythms

Repetitive oscillations in hormone secretion occurring every $24\,h$, typically responsive to light and dark.

Permissiveness

A regulatory relationship where one hormone must be present in sufficient amounts to 'permit' another hormone to have its full effects.

Synergy

When the combined actions of several hormones are greater than the sum of their separate effects.

Antagonism

A relationship where one hormone causes the loss of another hormone's receptors.

Nuclear receptors

Intracellular receptors used by lipophilic hormones to induce gene transcription in the cell nucleus.

Membrane receptors

Receptors in the target cell membrane used by hydrophilic hormones to activate receptor-enzyme complexes or second-messenger systems.

Carrier proteins

Proteins that lipophilic hormones are (loosely) bound to as they circulate in the blood.

Deiodinase

An enzyme in target cells that chemically modifies Thyroxine ($T_4$) to produce the more active Tri-iodothyronine ($T_3$).

5\alpha reductase

An enzyme in target cells that chemically modifies Testosterone to produce the more active Dihydrotestosterone (DHT).

Hyposecretion

An endocrine disorder where too little hormone is secreted by the endocrine gland.

Hypersecretion

An endocrine disorder where too much hormone is secreted by the endocrine gland.

Endocrine pancreas

A dedicated endocrine gland that secretes Insulin, glucagon, and somatostatin.

Adrenal gland

A dedicated endocrine gland that secretes Epinephrine, norepinephrine, Cortisol, Aldosterone, and Dehydroepiandosterone sulfate (DHEAS).