A histological and functional study was made of the thyroids of foetal guinea-pigs rendered goitrous by the administration of propylthiouracil (PT) to the mothers.
1. The thyroids of 30-day-old foetal guinea-pigs from PT-treated mothers were of the same weight and histological appearance as the glands of control foetuses.
2. Increased thyroid weight, loss of colloid and increased cell height were well marked in 36-day-old foetuses from treated animals.
3. The growth rate of foetal goitres was many times greater than that of corresponding maternal goitres as judged by thyroid weights and number of mitoses in 43- and 60-day-old foetuses. Expansion and folding of the follicular walls rather than increased follicle formation characterized the foetal goitres.
4. The ratios of thyroid/plasma inorganic 131I (T/Pl ratios) in 36-day-old normal and goitrous foetuses were of the same order of magnitude as found in adult animals. No significant difference was found between the T/Pl ratios of normal and goitrous foetuses and normal and goitrous pregnant animals.
5. Positive autoradiographs were obtained in 28-day-old normal foetuses and onwards. Individual thyroid uptakes (of 131I) of normal 60-day-old foetuses exceeded maternal uptakes. Goitrous foetuses showed a 'rebound' high uptake 3 days after the withdrawal of PT from the mother.
6. Untreated guinea-pigs born to mothers treated with PT were found to have true colloid goitres at ages of 4 and 8 months. Thyroid weight and average follicle size were bigger, average follicle cell height smaller than in control animals of the same ages. 131I uptakes were equal.