Endocrine System for the MCAT: Everything You Need to Know
/Learn key MCAT concepts about the endocrine system, plus practice questions and answers
(Note: This guide is part of our MCAT Biology series.)
Part 1: Introduction to the endocrine system
Part 2: Classifying hormones
a) Peptide hormones
b) Steroid hormones
c) Tyrosine derivatives
Part 3: Endocrine glands and secreted hormones
a) Pancreas
b) Anterior and posterior pituitary glands
c) Thyroid and parathyroid glands
d) Adrenal cortex and adrenal medulla
Part 4: Hormonal axes
a) Hypothalamic-pituitary-adrenal (HPA) axis
b) Hypothalamic-pituitary-gonadal (HPG) axis
Part 5: High-yield terms
Part 6: Passage-based questions and answers
Part 7: Standalone questions and answers
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Part 1: Introduction to the endocrine system
The endocrine system works with the nervous system to alter metabolism, guide reproduction, and regulate growth. In this guide, we will provide you with an overview of endocrine-based content you need to know for the MCAT. At the end of this guide, there is an MCAT-style practice passage and standalone questions to test your knowledge and show you how the AAMC likes to ask questions.
The MCAT requires memorization of multiple hormones, glands, and target cells (also called effectors). We’ll be presenting this information in a few different ways. When studying, it may be helpful to group hormones together based on glands that secrete them.
Let’s get started!
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Part 2: Classifying hormones
When functioning correctly, the endocrine system maintains homeostasis in the body. In other words, hormones are released in response to a condition rather than to create one. For example, insulin is released in response to high blood sugar levels; it is not released to create low blood sugar. This mechanism of action is referred to as negative feedback. While positive feedback is a mechanism to stimulate a condition through an action, negative feedback seeks to lessen a condition through an action.
How does the endocrine system accomplish this? The endocrine system releases hormones into the body’s general circulation, most often through blood, to affect other cells or tissues at a distance. (This is in contrast to autocrine signaling, in which a signaling molecule is released to affect the same cell or tissue, and paracrine signaling, in which a signaling molecule is released to affect a neighboring cell or tissue.)
Hormones can be classified into three distinct types, based on their structure: peptide hormones, steroid hormones, and tyrosine derivatives.
a) Peptide hormones
Peptide hormones are derived from peptides: short amino acid chains linked together by peptide bonds (e.g., O=C-NH bonds). Peptides are essentially short proteins. Therefore, peptide hormones are generally polar and water-soluble, allowing them to move freely through the bloodstream. However, peptides cannot easily diffuse through the nonpolar cell membrane. Therefore, peptide hormones generally require the presence of a receptor on the surface of the cell in order to effect an action.
When the peptide hormone binds to the receptor, there are multiple ways the receptor may respond. The receptor may act as an ion channel or interact with other proteins to create an ion-channel effect. (You can find more information on this signaling in our guide on lipids and cell membranes.)
The MCAT will most likely test your knowledge of a specific receptor system known as a G-protein coupled receptor, or GPCR. Upon binding to the hormone, G protein-coupled receptors undergo a conformational change that releases a G-protein by exchanging a low-energy GDP for a high-energy GTP. The G-protein activates the secondary messenger “cascade,” allowing secondary messengers like cAMP or IP3 to initiate a “cascade” of reactions within the cell. These secondary messengers serve to greatly amplify the effect of the hormone, as the binding of a single hormone molecule on the surface of the cell can result in a much higher concentration of secondary messengers to be activated!
Figure: An overview of GPCR function.
Here is a list of the specific peptide hormones you’ll need to study for the MCAT. We’ll cover their functions later in this guide.
| Anterior pituitary | Posterior pituitary | Parathyroid | Pancreas | Thyroid |
|---|---|---|---|---|
LH ACTH TSH Prolactin Endorphins Growth Hormones |
Oxytocin |
Insulin |
b) Steroid hormones
Steroid hormones are derived from cholesterol and are considered lipids. Recall that lipids are nonpolar and so are not soluble in water or blood. In the bloodstream, steroid hormones require a protein transport molecule to carry them.
In contrast to peptide hormones, they are able to directly diffuse through cell membranes. As a result, they do not require the presence of a receptor system on the cell membrane to effect their action. Steroid hormones directly pass through the membrane and can bind to receptors in the cytosol, be transported to the nucleus, and affect transcription by binding to DNA.
Organized by their glandular origin, here are the steroid hormones you’ll need to study for the MCAT.
| Adrenal Cortex | Gonads (Testes and Ovaries) |
|---|---|
Aldosterone *More detailed coverage in our guide on the digestive and excretory systems |
Testosterone Progesterone *More detailed coverage in our guide on reproduction and development |
c) Tyrosine derivatives
Tyrosine derivative hormones are also lipid soluble and require a protein transport molecule to be carried in the blood. As you might guess, they are modified versions of the amino acid tyrosine. This means that they are very small hormones in contrast to peptide and steroid hormones.
Organized by their glandular origin, here are the tyrosine derivative hormones you’ll need to study for the MCAT.
| Thyroid | Adrenal Medulla |
|---|---|
T4 (Contains 4 iodine atoms) |
Norepinephrine (Both categorized as catecholamines) |
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