A high variability between resting heart rate and working heart rate is generally associated with good health and low risk of heart disease. That is, it's better to rest at 40 and go up to 160 while exercising than to rest at 80 and go up to 160. RHR is fairly variable between folks, and trainable.
As long as your blood pressure is nominal (not so low you faint frequently -- sometimes a problem in elite endurance athletes with RHR in the 30s), and you feel fine, I wouldn't be bothered by that.
However, if you have other symptoms of hypothyroidism (weight gain, hair falling out, brittle nails and cracked skin, chronic tiredness, low body temperature and extremity temperature to name a few), a low RHR can be one symptom of many, but doesn't mean anything on its own.