Q During recent running I found that I lacked energy, and even though my heart rate was sky-high, I seemed to be plodding around every run. I went to my GP, and he diagnosed an underactive thyroid and prescribed Thyroxine. After a month, however, nothing seems to have improved. Will my running ever get back to normal?

A An underactive thyroid is not something that affects a patient overnight. Over a period of months, the gland slowly becomes less active, leading to a condition known as myxoedema. This illness is characterised by lethargy, weight gain, dry skin and hair, a slow pulse and sometimes a change in facial features. Trying to run while it is left untreated demands very great willpower.

Initial treatment with Thyroxine must be built up slowly. Treating an underactive thyroid too rapidly may cause problems with your heart (eg palpitations), and it may well take several months before the optimal dose is found.

You describe your training as plodding, and this is not an easy groove to get out of once established. You should certainly be trying to include some speed sessions in your training, even if it’s only some 100m strides. It may also help to train with somebody faster every now and then, to keep you from falling into one slow pace. Running will become easier as your metabolism returns to normal and the Thyroxine starts to work properly.

Finally, given your condition I would not be too concerned about your pulse rate. However, if you feel that it’s higher than it has been for a similar pace and effort, it might actually be worth leaving your heart rate monitor at home for a few weeks. That way you can avoid the psychological distress of failing to balance your pulse rate with your workload.

—How long does it take to walk a mile, RW Medical Advisor