Epidermoid cysts in the thyroid glands of male rats treated neonatally with oestrogen

Epidermoid cysts developed most frequently in the thyroid gland of male rats which had been castrated on the day of birth and injected with oestradiol benzoate (OB) for 30 successive days from birth. The incidence of epidermoid cysts was 90% in this group. The daily doses of OB were 1 μg during the first 10 days, 2 μg during the next 10 days, and 4 μg from days 21 to 30, respectively. These metaplastic lesions were not reversible after cessation of OB treatment. When OB was injected into intact male rats for the first 30 days of life, the incidence was decreased to 30%. In contrast, there was no manifestation of epidermoid cyst formation in the glands of control rats such as in intact male rats which had been treated with OB from days 31 to 60, or neonatally castrated and intact male rats, both given oil only.