There is a strong family history of thyroid disease from my mother's side. As far as I know the only other diabetic in my family was my Dad who was diagnosed T2 in his 70's. I had an over-active thyroid in my teens, resolved by 90% thyroidectomy age 18, then fine till age 41 when, diagnosed with under-active thyroid. I have now been on on a fairly stable dose of levothyroxine for almost 17 years.
I was diagnosed with T2 diabetes 7 years ago. At the time this made sense, as a fat middle-aged person with a diabetic parent. Initially I did reasonably well on a low carb diet, lots of exercise and then metformin; I never had the miraculous remission others have found, but did well enough to be congratulated as an exemplar by my GP practice. Last year things began to deteriorate, I simply couldn't get my blood sugars into single digits; I had lost a third of my body weight very slowly and deliberately over 7 years and finally got to the point where I no longer wanted to lose any more weight however I found I kept losing weight and my blood sugars just kept going up....additional diabetes meds did nothing, I started insulin in July and have subsequently been diagnosed with LADA.
When I was first diagnosed with diabetes I read as much as I could and one phrase stayed with me - something like "patients with thyroid disease tend to end up on insulin earlier than others" (if anyone else knows who wrote this I would love to know as I have not been able to find that quote again). At the time I thought I was T2 and hoped I could avoid this through diet and exercise choices, however it has proved to be my own personal experience.
Obviously my story is a single personal history, and we are all different. The original poster's question was about cause and effect. In my case I think I can feel confident that thyroid disease predated diabetes and that there is an association, but whether I would have gone on to develop diabetes without the additional genetic risk and/or the weight gain I had in mid-life I am not sure. I guess I will also never know how much of my mid-life weight gain was due to thyroid disease and how much due to diabetes. My personal experience is that hyperthyroid disease is far more difficult to manage than hypothyroid disease.
Essentially the whole business sucks.....wishing you all well xx