I'm sorry, I can't find anything that shows normal insulin levels at 20 mg/day, though I'm well aware of insulin resistance, because I have it. Insulin resistance is, typically, determined by fasting blood glucose levels and the hemoglobin A1c test.
The first thing I'd question is whether or not your thyroid levels are "well controlled" or simply "in range"... There's a huge difference. Because of the people we see in the Thyroid Community whose doctors say their thyroid levels are "fine", I have a particular aversion to that term. I'd have to see your exact thyroid hormone levels with reference ranges in order to agree that they are "fine".
Normal insulin levels are about 20mg a day to regulate blood sugars I produce 570mg a day which is more then 400times the healthy amount. But my blood sugar levels are always perfectly normal I've had this for more then 10 years and I've been on the metformin before. It's called insulin resistance which means my body has become resistant to insulin so my body keeps making more because it doesn't recognize it. If gone untreated for a long time it can become type 2 diabetes.
Insulin resistance is a known side effect of hypothyroidism which has been well controlled for the past decade with Synthroid .75mg I have been on both the natural armor and the cytmel meds they didn't make my T3 and T4 levels and better so I went back to synthroid it's like $5 a month for those meds. I get blood work and full ultrasounds done every 6 months and my endocrinologist is sure my thyroid levels are just fine.
Metformin is used all the time in people with insulin resistance all the drug dose is teaches the body to properly dispose of the unneeded insulin. I know because I've been on the drug twice before one time it worked the second it made me very sick. It's normal for people with insulin resistance to have high levels but according to my doctor anything over 500 is very dangerous and indicates a insulin producing tumor. Now my doctor is thinking something much worse but he doesn't want to talk about it he wants the endocrinologist to talk to me but like I said may take weeks to see him I live in Canada it's free to see a doctor but can take a while because first come first served.
What's the reference range for the insulin test? Ranges vary lab to lab and have to come from your own report.
At first glance, it would appear that you're producing adequate insulin, which is keeping blood sugar levels in check.
What are your fasting blood sugar levels? Have you had A1c tests? Typically, blood sugar levels determine the need for metformin, rather than insulin levels.
If you have hypothyroidism, what are your actual thyroid hormone test results? You should be getting tested for Free T3, Free T4 and TSH regularly.. Are you on a thyroid replacement hormone? If so, which one, what dose and for how long?