Insulin is a hormone produced in the pancreas that controls the amount of glucose in the bloodstream at any given moment. Insulin levels can be measured through a blood test, and there is no scientific consensus on optimal insulin levels. However, some studies have established an insulin reference range for a given population or defined a healthy insulin cutoff for the purpose of studying a related illness. Here are some examples of normal insulin levels:

• A study of 1434 Chinese men found that insulin levels ranged from 1.57 to 16.32 μU/mL, with a median value of 5.79 μU/mL.
• A small study that tested blood samples from 253 people concluded that the upper reference limit of insulin is less than 15 μU/mL.
• A study of 248 Indian participants set the upper limit for normal insulin levels at 17 μU/mL.
• According to the physician resource Medscape, normal fasting plasma insulin (a common measure of blood insulin levels) should stay under 25 microunits per milliliter (μU/mL) .
• Five Levels advisors suggest that fasting insulin should stay below 10 μU/mL.
• A normal fasting insulin level is 5 µIU/mL (micro international unit per milliliter) though ideally, this number should be around 3 µIU/mL.
• Optimal fasting insulin level is 2.6 - 24.9 uIU/ml.

Its important to note that different sources may have slightly different ranges, and the "danger zone" for insulin levels is still a point of contention among medical experts.