The principal aim of thyroid hormone replacement in patients with thyroid cancer is to maintain the TSH at a low and generally suppressed level. Several studies clearly demonstrate that a suppressed TSH correlates with improved survival in patients with well differentiated (papillary and follicular) thyroid cancer Associations of Serum Thyrotropin Concentrations with Recurrence and Death in Differentiated Thyroid Cancer. J Clin Endocrinol Metab. 2007 Apr 10; [Epub ahead of print]. Conversely, among patients with thyroid nodules having surgery, the level of TSH correlates modestly with the likelihood of having a final diagnosis of thyroid cancer after surgery, as outlined in Higher serum thyroid stimulating hormone level in thyroid nodule patients is associated with greater risks of differentiated thyroid cancer and advanced tumor stage J Clin Endocrinol Metab. 2008 Mar;93(3):809-14
Accordingly, thyroid cancer patients will require doses of thyroid hormone that are often somewhat higher than those rusually equired simply to keep the TSH in the normal range in the patient being treated with hypothyroidism. Following radioactive iodine, L-thyroxine is started, often the next day, and a TSH and thyroglobulin are obtained about 4-6 weeks later to ensure that they are both low. Occasionally patients will also receive T3 (Cytomel) for a week or two after the radioactive iodine treatment. There have been no randomized studies to determine whether we should aim for a TSH that is undetectable, versus a TSH of 0.1 or 0.2, and the precise TSH target level should be a matter of discussion between physician and patient. This area remains a matter of debate and discussion in the endocrine community, as illustrated in J Clin Endocrinol Metab 1999 Dec;84(12):4549-53 Levothyroxine suppression of thyroglobulin in patients with differentiated thyroid carcinoma.
A useful parameter that is helpful in guiding therapy is the thyroglobulin (Tg) level. If the Tg test is easily detectable when the TSH is 0.3, and falls to an undetectable level when the TSH is 0.1 or lower, this provides information regarding the sensitivity of remaining thyroid cells to TSH suppression.
At times, patients may have difficulty tolerating the doses of thyroid hormone required to keep the TSH suppressed. In these instances, the physician and patient should strive to achieve a level of TSH that keeps the patient healthy and symptom free, yet is not too high so as to engender concern about an increased risk of thyroid cancer recurrence.
If osteoporosis is a concern, then bone density may be measured at periodic intervals and standard measures of osteoporosis prevention and treatment (diet, exercise, smoking cessation, medications) should be considered.
Although a small study suggests that patients on long term thyroxine may exhibit modest abnormalities in parameters of arterial elasticity and left ventricular thickness as assessed by echocardiography, the clinical significance of these observations remains uncertain Long-term thyrotropin-suppressive therapy with levothyroxine impairs small and large artery elasticity and increases left ventricular mass in patients with thyroid carcinoma. Thyroid. 2006 Apr;16(4):381-6