The effect of recombinant human TSH (rhTSH) on thyroid function and ultrasonically determined thyroid volume was investigated in nine healthy euthyroid male volunteers. Each received either 0.9 mg rhTSH or isotonic saline in a randomized order, and thyroid volume and function were closely monitored during the following 28 d. No significant changes were observed after saline injection. After rhTSH stimulation, the median serum TSH increased from 2.03 mU/liter (range, 0.99-3.07 mU/liter) to more than 200 mU/liter (range, 78.9 to >200.0 mU/liter) after 4 h, with a subsequent rapid decline. Mean (+/-SEM) serum free T(4) and free T(3) peaked at 48 h with levels 204.7 +/- 26.1% and 226.9 +/- 31.4%, respectively, above baseline (P < 0.001). Twenty-four hours after rhTSH stimulation, mean (+/-SEM) thyroid volume was significantly increased by 23.3 +/- 5.8% (P = 0.003) and after 48 h by 35.5 +/- 18.4% (P = 0.02). On d 4 the mean thyroid enlargement had reverted to baseline values. One individual developed a 90-ml tender thyroid enlargement (initially 21 ml) 36 h after rhTSH administration, associated with a very high level of serum thyroglobulin. It is concluded that 0.9 mg rhTSH may result in a profound stimulation of not only thyroid function but also of thyroid size, appearing in the period 1-4 d after injection. Further dose-response studies are needed to clarify the potential hazards before routine use, for example in the context of (131)I therapy and goiter.