True hypoglycemia is indeed a rare disorder, in a person without diabetes (who is not on medications that are capable of lowering blood glucose below normal).
Proof of hypoglycemia is a venous plasma glucose level of less than 65 mg/dl. Ideally symptoms need to be present along with improvement of symptoms following treatment with glucose source like food. This is known as the Whipple's triad.
Once this is documented, one can proceed to determine the cause. This usually entails performing a 'prolonged fast test' that requires fasting for upto 48-72 hours, until the blood glucose level drops below 55 and symptoms occur, at which point blood samples are taken for sophisticated blood tests like Insulin, C peptide etc

Occasionally, a condition called Reactive hypoglycemia can occur. it is a complex and sometimes vague entity that is associated with symptoms mimicking hypoglycemia but often there is no true hypoglycemia. It is believed to occur from a rapid decline in blood glucose levels. Typically this is preceded by an excursion in blood glucose such as following a carbohydrate XXXXXXX meal.

Evaluation of this condition can be challenging and is best dealt with by an endocrinologist in person. The thyroid can contribute to low glucose only if it is severely under-treated or untreated.

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There is something called a mixed meal test which is again a fairly complex test to understand and perform, and hence is best conducted under an endocrinologists' supervision and guidance. You may wish to request your endocrinologist to consider doing this, per his or her discretion

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