An interactive guide to the glands that make hormones, what each hormone does, and the feedback loops that keep them in balance. Free to use in classes, patient education and health-center resource pages.
By PrescribedRXUpdated September 2026Reading level: intro college / high school A&P
Explore the glands
Select a gland on the body map or in the list to see where it is, what it makes and what happens when levels are off.
Location
Main job
Too much
Too little
How hormones stay in balance
Most hormones are controlled by negative feedback. When a hormone rises, it signals the glands upstream to slow down, much like a thermostat switching off the furnace. These three loops explain a lot of everyday lab results.
HPG axis
Testosterone
The hypothalamus releases GnRH in pulses.
GnRH tells the pituitary to release LH and FSH.
LH tells the Leydig cells in the testes to make testosterone. FSH supports sperm production.
Testosterone (and estradiol made from it) signals the hypothalamus and pituitary to release less GnRH and LH.
Why it matters: testosterone taken from outside the body also switches this loop off, so LH, FSH and the body’s own testosterone production fall. Medicines that block estrogen’s feedback at the hypothalamus and pituitary work the other way and raise LH and FSH.
HPT axis
Thyroid hormone
The hypothalamus releases TRH.
TRH tells the pituitary to release TSH.
TSH tells the thyroid to make T4 and a smaller amount of T3. Tissues convert T4 into active T3.
T4 and T3 signal the hypothalamus and pituitary to release less TRH and TSH.
Why it matters: a high TSH usually means the thyroid is underactive, because the pituitary is pushing harder. A low TSH usually means it is overactive.
Gut and pancreas
Blood sugar
Food in the gut triggers L-cells in the intestine to release GLP-1, an incretin hormone.
GLP-1 boosts insulin release from pancreatic beta cells when glucose is high and lowers glucagon.
Insulin moves glucose into muscle, fat and liver cells, so blood sugar falls.
GLP-1 also slows stomach emptying and acts on the brain to increase fullness.
Why it matters: GLP-1 receptor agonists such as semaglutide, and dual GIP/GLP-1 agonists such as tirzepatide, mimic these gut hormones. That is why they lower blood sugar and appetite.
Reference points from common lab tests
Labs use different methods, so always compare a result with the range printed on your own report.
Test
Reference point
What it tells you
Source
Total testosterone (men)
264–916 ng/dL
Harmonized range for healthy, non-obese men aged 19–39 (2.5th to 97.5th percentile), measured in the morning.
Travison et al., 2017 [4]
Hemoglobin A1C
< 5.7%
Normal average blood sugar over the past 2–3 months.
CDC [5]
Hemoglobin A1C
5.7–6.4%
Prediabetes.
CDC [5]
Hemoglobin A1C
≥ 6.5%
Diabetes.
CDC [5]
TSH
Lab-specific
High usually points to an underactive thyroid, low to an overactive one.
NIDDK [2]
All glands at a glance
Gland
Location
Key hormones
Main job
Use this guide
Teachers, librarians and health-center staff are welcome to link to this guide from course pages, LibGuides and student resource lists. No sign-up or permission needed.
Cite it (APA)
PrescribedRX. (2026, September). The endocrine system: An interactive guide to glands and hormones. https://prescribedrx.com/endocrine-system/
Link to it
<a href="https://prescribedrx.com/endocrine-system/">The Endocrine System: An Interactive Guide (PrescribedRX)</a>
Sources
MedlinePlus, U.S. National Library of Medicine. Hormones.
National Institute of Diabetes and Digestive and Kidney Diseases. Endocrine Diseases.
Travison TG, Vesper HW, Orwoll E, et al. Harmonized reference ranges for circulating testosterone levels in men of four cohort studies in the United States and Europe. J Clin Endocrinol Metab. 2017;102(4):1161–1173. doi:10.1210/jc.2016-2935.
