Gallstone problems can arise for a number of issues. Firstly merely being overweight puts you at a higher risk. Losing weight quickly puts you at a higher risk. However the Cambridge Diet in itself will not cause them due to the balanced minerals and vitamins that ensure you get everything you need. So losing the amount of weight that you do with this diet can be a cause of gallstones, but it is not very common, at all. Also gallstones are not something that is generally life threatening. The majority of people actually never know they even have them. And if you do they often pass with little problem, more slight discomfort. The cases where it is more serious are not very common, and so the chances of this happening because of the diet are fairly slim.
And to the second part about thyroid issues... there is no proof that diet or specific foods affect hypo or hyperthyroidism. Though there is a some research that suggest that certain deficiencies such as iodine can cause them, however fixing the deficiency fixes the thyroid problem! But again, because it's CD is specially balanced to give you everything you need, that won't happen. A thyroid problem is a physical chemical problem within your body to do with the thyroxine that your thyroid gland produces and TSH which is the chemical your brain produces to tell your thyroid to produce more or less thyroxine. Because of how it is this chemical imbalance, it's not something that is caused by dieting. In fact, as you lose weight it will actually help improve thyroid problems if they exist already, but it won't cause it.
Wow I've just realised how much I ramble, sorry!! I've probably really not explained that very well at all, but I hope it makes some sense! If not let me know and I'll try and explain it better, I'm not always very good at explaining things.