Atlas MCAT Course
Systems & Physiology → Growth & Metabolic Hormones
Growth & Metabolic Hormones
Setting long-term fuel use and body state.
01The idea
You already know that insulin and glucagon control short-term metabolic state, and that neural signals control immediate actions. But some hormones operate on a longer horizon: days to months, governing growth, size, baseline metabolism, and how tissues respond to other signals.
They tune the system, not the switches.
THREAD · Regulation vs. capacity
02Build the model
AGrowth hormone (GH): permissive, not direct
Growth hormone is often misunderstood. GH does not directly cause most growth; it acts broadly on many tissues and alters how tissues respond to nutrients.
Its functional role: signal long-term nutrient sufficiency, promote growth, repair, and remodeling, and shift metabolism toward fuel availability for growth.
BGH axis logic (no memorization needed yet)
The control pattern is hypothalamus → pituitary → peripheral effect, with feedback from downstream signals stabilizing output. GH’s major downstream mediator is insulin-like growth factor (IGF).
This preserves specificity while maintaining systemic coordination.
CMetabolic effects of growth hormone
GH reduces glucose uptake in some tissues, promotes fat utilization, and preserves glucose for growth-critical processes. This can feel counterintuitive until you reframe it:
DThyroid hormones: setting the metabolic “idle speed”
If GH sets growth capacity, thyroid hormones set metabolic rate. Thyroid hormones increase basal metabolic rate, oxygen consumption, and heat production. They do not directly trigger pathways, act quickly, or cause short-term fuel shifts.
EWhy thyroid hormones act on gene expression
Thyroid hormones are lipid-soluble, enter cells, and alter transcription. This explains their slow onset, long duration, and whole-body effects.
FThyroid axis: feedback logic applied
Using axis reasoning: low thyroid hormone → upstream signals increase; high thyroid hormone → upstream signals decrease. Symptoms emerge from altered metabolic rate, not from discrete pathway failures. This makes thyroid disorders highly predictable conceptually.
GInteraction with insulin and glucagon
Growth and thyroid hormones do not replace insulin or glucagon; they modify how tissues respond to them. Example logic: the same insulin signal, with a different baseline metabolic rate, gives a different outcome.
HTime-scale comparison (important MCAT framing)
Understanding this comparison solves many “why doesn’t X happen immediately?” questions.
03See the connections
The lesson names these links itself. Read the map left to right.
Growth hormone
- Permissive
- IGF executes locally
- Spares glucose, uses fat
Thyroid hormones
- Set basal metabolic rate
- Gene expression: slow, lasting
Layering
- Tune how tissues respond to insulin and glucagon
- Days to weeks
Next: stress hormones.
04Reason with it
Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.
Think first.
What mainly carries out growth hormone’s growth effects in tissues?
Think first.
A patient has low thyroid hormone. What do you predict?
Think first.
How do thyroid hormones differ from insulin?
Think first.
A thyroid hormone dose is given. Why don’t effects appear within minutes?
05MCAT lens
Predict the direction
High GH → increased growth potential, altered fuel use
Low thyroid hormone → reduced metabolic rate
Thyroid excess → increased heat, fuel turnover
GH effects → indirect, delayed, systemic
No lists required.
06Common traps
Trap 1: GH directly causes tissue growth
Misconception
IGF mediates most effects.
Correct model
GH enables growth programs.
Trap 2: Thyroid hormones act like insulin
Misconception
They tune rate, not direction.
Correct model
Baseline vs switching.
Trap 3: Endocrine effects are immediate
Misconception
Many are deliberately slow.
Correct model
Time scale matches function.
07Dependency check
Can you reconstruct this concept without looking at the page? You should now be able to reason through:
Why some hormones act indirectly
Check against 02 · A and B: GH is permissive; IGF executes.
Why gene-level regulation is slow but powerful
Check against 02 · E: lipid-soluble hormones change transcription.
How growth and metabolism intersect
Check against 02 · C: GH shifts fuel use to keep growth supplied.
Why baseline tuning matters
Check against 02 · G: same insulin signal, different outcome.
If something feels unclear, the missing idea is usually thisSome hormones don’t change what happens — they change how strongly and how fast everything happens.
08Remember
REMEMBER · if you keep only a few things
- GH and thyroid hormones tune the system; insulin and glucagon flip switches.
- GH is permissive; IGF does the local work.
- GH spares glucose and promotes fat use.
- Thyroid hormones set basal metabolic rate, O₂ use, and heat.
- Gene-expression hormones are slow and lasting.
- Time scales: ms (neural), min–hrs (insulin/glucagon), days–weeks (GH/thyroid).
09Forward hook
We’ve covered metabolic hormones, growth regulation, and baseline metabolic tuning. The next major endocrine role is adaptation to stress and challenge.
How does the body adapt when demand exceeds its baseline capacity?
That leads directly to adrenal hormones, acute vs chronic stress, and interaction with metabolism and the ANS.
Next lessonStress Hormones & the Adrenal System→