Can't lose weight with a underactive thyroid. i am doing plenty of exercise via mountain biking but my weight seems to yo yo, any advise?

Thyroid medicine: I hope you are taking thyroid hormone replacement therapy and having your doctor check your lab results every 3-6 months. That you are mountain biking regularly suggests that you are taking your thyroid meds. Your "yo yo" weight may reflect your mountain biking converting fat to muscle which usually happens without change in weight.

Thyroid and weight: If your thyroid medications are properly adjusted, such that your TSH and ft4 are normal, then your metabolism should be appropriate for your age and gender. Perhaps you should focus more on your overall fitness, and not your weight, per se. With all the mt biking, you must be fairly fit, and maybe gaining muscle mass.