Biology · Endocrine system
Tropic vs non-tropic hormones, and the FLAT PEG trick
Some hormones do a job themselves. Others mostly tell another gland to do its job. Those are tropic hormones. By the end of this you'll be able to sort any anterior pituitary hormone into the right group, and use the chain to find where a hormone problem is.
By Boardling, from the Instaboard teamLesson 002 · 3:13 videoUpdated
Tropic vs non-tropic hormones in 30 seconds
- A tropic hormone's main target is another endocrine gland. A non-tropic hormone acts straight on the tissue that responds.
- Tropic (anterior pituitary): TSH → thyroid, ACTH → adrenal cortex, FSH and LH → ovaries and testes.
- Not tropic: prolactin (milk), endorphins (pain), growth hormone (mostly direct, but also triggers IGF-1 from the liver).
- FLAT PEG: F, L, A, T are tropic (FSH, LH, ACTH, TSH); P, E, G are not (prolactin, endorphins, GH).
- Hypothalamic releasing hormones (TRH, CRH, GnRH) are tropic too: they act on the anterior pituitary.
The notes board from the lesson
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What tropic means
In the video at 0:14
Tropic means "acting on". A tropic hormone's main target is another endocrine gland. It doesn't change your metabolism or growth directly: it switches on a gland, and that gland makes the hormone that does the work.
A non-tropic hormone skips that step and acts straight on the tissue that responds.
If it helps, think of the hypothalamus as the boss and the anterior pituitary as the manager. The manager doesn't do the work; it tells the glands when to work.
The four tropic hormones
In the video at 0:44
TSH, thyroid-stimulating hormone, targets the thyroid, which makes thyroid hormone.
ACTH, adrenocorticotropic hormone, targets the adrenal cortex, which makes cortisol.
FSH and LH target the gonads, the ovaries and the testes. They drive the sex hormones and the making of eggs and sperm.
The ones that aren't tropic
In the video at 1:09
Prolactin acts straight on the breast to make milk. Endorphins act on the nervous system to dull pain.
Growth hormone is the tricky one. It acts on many tissues directly, but it also tells the liver to make a second hormone, IGF-1. So most textbooks file it as non-tropic, with an asterisk.
The FLAT PEG trick
In the video at 1:35
F, L, A, T: FSH, LH, ACTH and TSH are tropic. P, E, G: prolactin, endorphins and growth hormone are not.
The hypothalamus, one level up
In the video at 1:48
The hypothalamus plays the same game one level up. Its releasing hormones, like TRH, CRH and GnRH, act on the anterior pituitary, so they're tropic too. That's why these chains are called axes, like the HPA axis and the HPT axis.
Where's the break?
In the video at 2:07
A chain tells you where to look when something breaks. If the thyroid itself fails, thyroid hormone drops and TSH climbs, because the pituitary keeps signalling a gland that can't respond.
If the pituitary fails, TSH stays low, and so does thyroid hormone. Rule of thumb: when the signal is high but the hormone is low, look at the gland that isn't answering.
Practice question
Cortisol is low, but ACTH is high. Where is the problem?
Show the answer
The adrenal gland (adrenal cortex). ACTH is high, so the pituitary is doing its job. The adrenal cortex just isn't answering.
Common mix-ups
Growth hormone is a tropic hormone like TSH.
Most textbooks file GH as non-tropic: it acts on many tissues directly, with IGF-1 from the liver as the asterisk.
Tropic hormones only come from the pituitary.
Hypothalamic releasing hormones (TRH, CRH, GnRH) are tropic too: their target is the anterior pituitary.
High TSH means the pituitary is overactive.
Usually it means the thyroid isn't responding, so the pituitary keeps signalling (with low thyroid hormone).
How this lesson was made
Boardling lessons are made by the Instaboard team. The narration is a synthetic (text-to-speech) voice, and the script and these notes were written with AI help, then checked against the sources listed below (last checked October 29, 2026). The notes board is built live in Instaboard during the video. Spotted a mistake? Tell us and we'll fix it.
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Questions
What is a tropic hormone?
A hormone whose main target is another endocrine gland. It switches that gland on, and the gland makes the hormone that does the work. TSH, ACTH, FSH and LH are the classic examples.
Which anterior pituitary hormones are tropic?
FSH, LH, ACTH and TSH (the FLAT in FLAT PEG). Prolactin, endorphins and growth hormone (PEG) are usually classed as non-tropic.
Is growth hormone tropic or non-tropic?
Most textbooks call it non-tropic because it acts directly on many tissues, but it also stimulates the liver to make IGF-1, so it's sometimes described as having tropic effects.
What does FLAT PEG stand for?
FSH, LH, ACTH, TSH (tropic) and prolactin, endorphins, growth hormone (not tropic).
Is TRH a tropic hormone?
Yes. TRH from the hypothalamus acts on the anterior pituitary to release TSH, so its target is another endocrine gland.
Exam fit
- MCAT 3A: endocrine system
- AP Biology Unit 4
- Anatomy & physiology
- A-level biology