The fetal thyroid dose for different conditions can be calculated using the model in HPJ Vol. 43 pp 573-582. The risk and hence a decision about the minimum time can only be made by the expectant mother and her doctor. John R Johnson, PhD Scientist Emeritus, HPD/PNNL MS K3 53 Box 999 Richland, WA, 99352 e:mail; jr_johnson@pnl.gov Phone: (509) 375-6850 FAX: (509) 375-2019