I found the comment below quite interesting. (Some of the rest of the advice on this page is rather dubious however)
Answer to question 7: 'The treatment with thyroxine can be for two purposes, either to suppress thyroid tissue or merely to treat hypothyroidism. It would be appropriate to suppress TSH in the case of a goitre or previous treatment for thyroid carcinoma. A low (subnormal) TSH may be due to pituitary damage, or may be due to excess production of thyroxine or T3, or excess intake of these agents. In the latter situation, the TSH is supressed. However, when one is trying to treat hypothyroidism, the ideal treatment would be to bring TSH down into the normal range, but not suppress it necessarily. Nevertheless, concerns which many physicians have expressed over the past few years about osteoporosis if TSH is suppressed by thyroxine have proven recently to be incorrect. Studies have shown NO reduction in bone mineral density, and no osteoporosis when thyroxine is taken even in suppressive doses. Only when patients have had actual Graves' disease -- "overactive thyroid," is there a risk of osteoporosis and even that risk is small. Nevertheless, one should strive for ideal therapy and the ideal for hypothyroidism is to have all tests of thyroid function normal.'