It is now recommended that levothyroxine doses be reduced when the drug is used to suppress thyroid-stimulating hormone (TSH), so that a thyrotoxicosis-induced increased risk for osteoporosis is minimized in patients with a history of thyroid cancer who have been apparently tumor-free for many years [1-5]. However, no data address whether low but detectable TSH levels can achieve the clinical benefit that was seen when TSH was extinguished. Furthermore, a tumor-free state is difficult to definitively ascertain.

We are concerned that some patients with a history of thyroid cancer may be harmed when TSH is unclamped unless they are first evaluated for dormant and unsuspected persistent or recurrent tumor. This concern was heightened by our recent discovery, incidental to a plan to reduce the levothyroxine dose, of unmanifested thyroid cancer that was metastatic to the lung in a 57-year-old woman with asymptomatic osteoporosis and TSH suppression. Thirty years before, papillary thyroid carcinoma had been treated surgically, followed by I¹³¹ scanning. Whole-body scans in 1965, 1973, and 1981 were negative. The current thyroid sonogram, chest radiograph, and magnetic resonance imaging scan were physiologic, and the test for thyroglobulin (border-line high level) was invalid because of interfering antibodies. Withdrawal of levothyroxine, hypothyroidism, and a whole-body I¹³¹ scan were needed to reveal the metastatic cancer (Figure 1).

View larger version (64K): [in this window] [in a new window]   Figure 1. Top. Whole-body scan 48 hours after 4.0 mCi of I131. Abnormal accumulation of the isotope is present in the posterior part of the right side of the neck (N), in the superior mediastinum just to the right of the midline (M), and in the right lung (RL). Normal activity in the blood pool outlines the body and the heart (H). Radioiodine is also present in the salivary glands, parotid gland (P), the paranasal sinus, the stomach (S), and the bowel (B). Bottom. Whole-body scan 1 week after therapy with 314 mCi of I131. The therapy dose was used as the tracer material. Radioiodine-labeled thyroxine in the circulation shows the body. The image of the liver (Li) reflects the enterohepatic circulation of thyroxine. Abnormal uptake is now seen in both lungs (L) and in a mass in the superior mediastinum (M). The increased uptake of the right side of the neck reflects the nodule in the back of the neck (N) and overlying salivary gland. Residual inorganic iodine can be seen in the salivary glands (SG), the stomach (S), the bladder (B), and the genital region below the bladder.

Because of the excellent prognosis when TSH is undetectable, the frequency of late recurrence or of prolonged dormancy of metastatic thyroid cancer has not been examined critically. This patient and other experiences have led us to conclude that each patient should be evaluated for evidence of hidden cancer before the levothyroxine dose is reduced. Reducing the dose without this evaluation could expose the thyroid cancer cells to TSH, the patient to an unknown but perceived increased risk for activating the cancer, and society to increased and probably unnecessary cost.