Due to the remarkable increase in thyroid cancer cases, the number of patients treated with radioiodine (I-131) shows a sharply increasing trend in recent years. Accordingly, radiation exposure of other people, particularly caregivers or comforters, after release of patients from hospitals is getting more attention than ever. In the present study, empirical equations are proposed for estimation of doses to caregivers. Only patients administered with therapeutic amounts of I-131 after thyroidectomy were considered. External radiation doses to 70 caregivers or family members were measured using thermoluminescence dosimeters (TLDs). The mean, external, effective dose to caregivers, during a nursing period of 5-9 d after patient quarantine for 3-4 d in the hospital, was 0.12 +/- 0.10 mSv. This is only 2.5% of the dose limit recommended by the International Commission on Radiological Protection for caregivers. By analyzing those individual doses to the caregivers, values of a factor affecting caregiver doses, K, are obtained for use in estimation of caregivers' doses. The factor reflects the degree of engagement of the caregiver to the patient, and hence it is named the engagement factor. The mean value of the engagement factor in this study was 1.3 +/- 0.88. With the help of the engagement factor, the total external dose to a caregiver can be estimated as 1.1 x Q(0) x e(-0.05Tr) mSv, where Q(0) is the administered activity of I-131 (GBq) and T-r is the patient's release time (h) after admistration of radioiodine. Based on the dose estimation model developed in this study, by comparing the cost of extended quarantine against that incurred by release of the patient, including the burden of radiation exposure of caregivers or family members, the reasonableness of current quarantine periods was revisited. It was found that the dichotomous policy (i.e., hospitalizing patients administered I-131 over 1.1 GBq for a period of 3-4 d compared with treating other patients administered below 1.1 GBq as outpatients) is unjustifiable; this is particularly true for those treated with a few GBq. Based upon the dose estimation model presented herein, tables suggesting an appropriate quarantine period depending upon the activity of the administered I-131 are provided for use as reference in deciding when to release patients treated with therapy levels of I-131 after thyroidectomy.