A study of the rat thyroid during perinatal days with observations of compensatory changes following unilateral thyroidectomy

Measurements of volume of the thyroid, diameter of follicles and height of follicular cells were made under normal or experimental condition of unilateral thyroidectomy in late fetal and newborn rats. During perinatal days, the thyroid did not enlarge, the diameter of follicles was reduced and the height of follicular cells lowered. Unilateral thyroidectomy in fetuses on day 20 of pregnancy caused two days later a significant increase in the ratio of the volume of remaining lobe of thyroid to body weight. The height of follicular cells was increased in a unilaterally thyroidectomized fetus when compared with that in a litter‐mate control fetus. In newborn rats, the unilateral thyroidectomy on day 1 of life also caused two days later a significant increase, though less than that observed in fetuses, in the ratio of thyroid volume to body weight. The height of follicular cells was not increased. The observations support the view that in the rat the pituitary‐thyroid system begins to function before birth and that the founctioning of the system is slightly reduced just after birth.