Role Of Recombinant Human

Thyroid Factor

The Natural Thyroid Diet

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The recent availability of clinical grade rhTSH coincided with the cessation of the production of bovine TSH (bTSH). The latter was used clinically to stimulate 131I uptake by metastatic thyroid cancer lesions; however, it had a number of side effects, which were sometimes severe. Clinical trials soon demonstrated that rhTSH is able to replace the need for patients with DTC to undergo T4 withdrawal to have whole-body 131I study and serum Tg testing, while eliminating the undesirable side effects of bTSH.23 This compound has been approved by the US Food and Drug Administration for use as an adjunctive diagnostic tool for serum Tg testing and 131I imaging in the follow-up of patients with DTC. However, it must be appreciated that there remains a small risk of missing a diagnosis of residual cancer or of underestimating the extent of disease with rhTSH-mediated testing and that thyroid hormone withdrawal testing remains the standard diagnostic modality, especially in high-risk patients.

The decisions whether to perform rhTSH-medi-ated testings and whether and when to withdraw a

Years followed

Figure 2-18. Cumulative survival curves by Kaplan-Meier plot for disease-free and cancer-specific survivals, respectively, in patients with differentiated thyroid carcinomas followed over a 25-year period. Reproduced with permission from Loh KC et al.15

10 15

Years followed

Figure 2-18. Cumulative survival curves by Kaplan-Meier plot for disease-free and cancer-specific survivals, respectively, in patients with differentiated thyroid carcinomas followed over a 25-year period. Reproduced with permission from Loh KC et al.15

patient from T4 are complex and require individual judgment. After the first negative post-131I therapy scan, and if the basal serum Tg is undetectable, we recommend the use of rhTSH for the second and subsequent scans to eliminate the need for T4 withdrawal and the attendant morbidity associated with hypothyroidism. Patients will continue on T4 treatment but will have to follow a low-iodine diet for 1 week prior to the study (Table 2-2). A positive scan or a rise in serum Tg to >2 ng/mL is indicative of metastatic disease, which would require high-dose 131I therapy.24 For 131I ablative therapy, the patient would require routine T4 withdrawal preparation.

If the 131I scan after rhTSH is negative, follow-up rhTSH studies can be done using serum Tg measurements alone. The protocol is the same as outlined in Table 2-2 except that the patient does not have to follow a low-iodine diet and no 131I is administered. A rise in serum Tg > 2 ng/mL following rhTSH is an indication for a withdrawal scan and possible treatment with 131I. The rhTSH-mediated Tg testing could be repeated annually for 3 to 5 years and thereafter at less frequent intervals.

For very high-risk patients, however, thyroid hormone withdrawal testing is currently the standard practice. This may be supplemented periodically with an ultrasonographic examination of the neck or spiral computed tomography or magnetic resonance imaging of the neck and chest to rule out metastases that do not pick up 131I or synthesize Tg. Whole-body positron emission tomography (PET) scanning with 18F-fluorodeoxyglucose (FDG) should be considered when tumor is suspected on the basis of high serum Tg levels but negative imaging studies.25 Metastatic masses that do not pick up 131I and are detected either with conventional imaging modalities or FDG-PET scanning may be amenable to surgical resection or external radiotherapy.

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