I'm concerned about my continued weight gain (around 6 kg in past 3 years)despite improvement in diet and exercise. Menapause &/or thyroid??

Thyroid / menapause. Please do see your doc who may check your thyroid or do blood work and take history to see about menapause. Lifestyle is also a factor to consider. Peace and good health.
Weight gain. Yes, you are correct in that it could be the thyroid or menopause. Those would be things that can be checked easily with your regular GP. I know it is frustrating and age does tend to make it harder to maintain or lose weight after 40!