Your patient was born in 1945 in Washington State. She grew up on a farm near the Hanford Nuclear Reservation, where her father worked as a machinist during World War II. The family history reveals that the woman lives with her husband of 41 years; her daughter, born in 1963, who is 6 months pregnant; and her daughter's husband. They have lived in your community for the last 12 years, in a single home in a low income area of town.

The patient's past medical history is noncontributory, and her family history is unremarkable. Her father died at age 84 of a myocardial infarction, and her mother died of colon cancer at age 77. The patient has no family history of thyroid disease or of other endocrine disease. She has two brothers and a sister; all are reportedly in good health.

Your patient does not have symptoms consistent with hyperthyroidism or hypothyroidism; does not smoke and has not smoked in the past, but she occasionally drinks alcohol. She does not have any past workplace or hobby chemical exposures, and she has not received any therapeutic radiation exposures.

1. Which organ system is considered the critical organ for exposure to I-131?
2. What are the main routes of human internal exposure for I-131?
3. What are the most significant health effects from exposure to I-131?
4. Which group is most at risk for health effects from exposure to I-131, and why?