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?

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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.