Testing Sequence

Thyroid Factor

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The testing sequence varies among clinicians (see Chapter 19), but the one we use is shown in Figure 20.3. In a patient with no uptake outside the thyroid bed on the RxWBS after 131I remnant ablation who is clinically free of disease, the first step is to perform a serum Tg measurement during TSHT. If the serum Tg is undetectable, neck ultrasonography is done and 0.9 mg of rhTSH is administered intramuscularly for 2 consecutive days; 72 hours after the last rhTSH injection a serum Tg is measured. This can be done with THW, but the TSH levels are less con-

Testing Sequence after Negative 131I DxWBS and High Tg

Patient Clinically Free of Tumor After Initial Therapy

Tg on Thyroid hormone Therapy*

Tg Undetectable

Ultrasonography & RhTSH-Tg 72 h after last injection

Long-term ^ Follow-up i

-Positive

Tg Detectable

Post-treatment WBS

Neck Ultrasonography

Positive

Consider

Surgery

Post-treatment WBS

FDG-PET

Negative

Positive

Figure 20.3 Sequence of tests during follow-up after total thyroidectomy and thyroid 131l remnant ablation when Tg is elevated and RxWBS is negative. Tg is thyroglobulin, rhTSH is recombinant human TSH-a administered at a dosage of 0.9 mg on 2 consecutive days with serum Tg measurement 72 hours after the last injection.* This may occur at any time during follow-up, but is more likely to be encountered immediately after initial therapy.t Urine iodine should be <100 mg/g creatinine, TSH should be >30 mlU/L following thyroid hormone withdrawal,lithium pretreat-ment should be considered. If the patient cannot become hypothyroid or the TSH will not rise in response to THW, rhTSH can be used to stimulate 131l uptake. t The differential diagnosis of this condition is detailed in the text.

sistent during withdrawal [45], making the Tg cutoff levels used for making therapeutic decisions more variable unless the TSH is >30 mIU/L [28]. If the serum Tg is less than 2 mg/L, the patient simply is followed at 6- to 12-month intervals with serum Tg measurements during THST. If on the other hand, the serum Tg rises above 2 mg/L after rhTSH stimulation and the neck ultrasonography is negative, 131I therapy should be considered, particularly if the Tg rises above 5 mg/L after rhTSH. The exact Tg level to consider 131I treatment is a matter of debate, but in general the higher the Tg level is at the time of treatment, the more likely 131I uptake will be seen on the RxWBS [14]. The levels we use to consider further study is a serum Tg level >2 mg/L after rhTSH and a level >5 mg/L after THW. If there is no uptake on the RxWBS, then an 18FDG-PET scan should be done.

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