A few things to consider looking at:
Thyroid function
- To have the thyroid properly checked, it would include all six of these tests: free T4, free T3, reverse T3, TPOAb and TgAb antibodies (to exclude Hashimoto autoimmune), TSH.
Blood sugar function
Important Blood Sugar and Insulin Resistance Measurements (all before breaking fast)
1. A1c. "Low" carb means something slightly different for everyone, but if your A1c is too high (5.3% or higher) you should try to lower carb intake as it can indicate excess sugar or carb intake.
2. Triglycerides. If your triglycerides are high (<100 mg/dL is optimal) you should try to lower your carb intake as your liver is transforming excess into energy storage.
3. Ketones. I also try to keep my ketone level around 0.5 or higher, but I never go to extremes and never add exogenous ketone salts either. Great indication of diet quality.
4. Insulin. Higher than 5 uIU/mL is an indicator of too high carb/sugar intake and/or start of insulin resistance
5. Blood glucose. <100 mg/dL is the expected norm, but <85 is better for longevity and health.
6. C-peptide. Indication of insulin production
Nutrient deficiencies
Have you checked for any malabsorption and nutrient deficiency issues? Something like Genova Diagnostics - FMV gives a great overall picture of nutrient deficiencies, neurotransmitter metabolites, microbiome dysbiosis including SIBO and candida, pancreatic enzyme issues, and many more. Great Plains Laboratories has organic acid and nutritient level tests. A comprehensive stool test can also give a wealth of info.
Also, have you taken a hard, honest look at everything you eat and drink? Do you know how many calories you need and take in? How many grams of carbs per meal and day? Have you kept a food diary to see if there are any correlations? Any supplementation or medicine would also help to keep track of to see whether they are having a negative impact.