Context: Recombinant human TSH (rhTSH) is used to evaluate thyroid carcinoma patients, and off-label for ¹³¹I thyroid ablation and nontoxic goiter therapy.

Objective: To determine the optimal time for ¹³¹I administration following rhTSH.

Participants: 25 euthyroid nongoitrous volunteers.

Design: baseline 24-hour thyroid ¹²³I uptake (RAIU) was measured, then 0.1 mg rhTSH was administered. ¹²³I was administered either 24, 48, or 72 h after rhTSH, and a repeat 24-hour RAIU was obtained.

Setting: academic research center.

Main Outcome Measures: Thyroid function tests, thyroid ultrasounds, and electrocardiograms (ECGs) were measured before, daily for four days, and seven days after rhTSH.

Results: Serum TSH concentrations 24 h after rhTSH increased from 1.7 ± 0.5 µU/ml (mean ± SD) to 13.3 ± 4. Twenty-four-hour RAIUs rose from 25 ± 5 to 47 ± 8% (88% increase) when the ¹²³I was given at 24 h after rhTSH; from 29.8 ± 7 to 40.5 ± 13% (36% increase) when the ¹²³I was given at 48 h; and were unchanged when the ¹²³I was given at 72 h. The post-rhTSH RAIU increase was greater at 24 than at 72 h (P < 0.005) and marginally greater than at 48 h (P < 0.57). Thyroid volumes significantly increased 48 h after rhTSH (10 ± 3.8 vs. 11.1 ± 3.7 ml, P < 0.009). ECGs were normal.

Conclusions: marked increases in RAIU occurred when ¹²³I was given 24 h after rhTSH administration to euthyroid volunteers. Smaller increases were observed at 48 h and none at 72 h.