Thyroid: Am I taking too much medication?

I have been diagnosed with an underactive thyroid gland, and now have to take thyroxine tablets every day. I was started on a low dose, and this has gradually been increased because blood tests still show my levels to be low. The starting dose was 50mcg a day, and I am now up to 300mcg a day. Is there a maximum dose that is safe? And could I take a dose as high as 400mcg a day, if it were necessary?

Dr Fred Kavalier answers your health question:

Thyroxine therapy for an underactive thyroid gland is a form of hormone-replacement therapy. If your thyroid is not producing enough thyroid hormone, you can take thyroxine, which is the synthetic form of thyroid hormone that is designed to replace what your body is unable to produce naturally.For most people with an underactive thyroid, it is relatively simple to find the right dose. Elderly people sometimes only need 50mcg (micrograms) a day (1,000mcg is the equivalent of one milligram). The most common dose is 100mcg a day, and occasionally I have seen a patient who needs 200mcg a day. The correct dose can be calculated by doing blood tests to look at the levels of thyroid hormone in the blood, and also the levels of thyroid-stimulating hormone (TSH). If you have too much thyroid hormone, TSH levels fall. If you are short of thyroid hormone, TSH levels go up as the body attempts to stimulate the thyroid gland to produce more hormone. Patients are considered to be on the correct dose if their TSH level is normal. If your blood tests show that you appear to need more than 200mcg, there may be something odd going on, which needs further investigation by an endocrinologist. One possibility is that your body is resistant to the effects of thyroxine.

Please mail your questions for Dr Fred to health@independent.co.uk. He regrets that he is unable to respond personally to questions.