There is a problem if one is on metformin ; the metformin seems to destroy TSH / or reduce it , but only in hypo thyroid persons, this will result in a too low TSH and a masking of hypothyroidism as if one had much higher levels as one really have.
So when on metformin TSH can not be a trusted guide alone and it seems to continue over time, my GP didn't know that and was busy lowering me time after time which I denied as I knew I was not hyperthyroid . Then I started to look for What the f.ck could be the matter and were lucky to stumble over that information from a large study
http://care.diabetesjournals.org/content/32/9/1589
http://www.medpagetoday.com/endocrinology/diabetes/47788