The effects of fetal thyroidectomy during the second trimester in the ovine fetus

Seven sheep fetuses surgically thyroidectomized (Tx) during the second trimester at 81 to 87 days' gestation were acutely studied near term. Compared to 10 age-matched, sham-operated controls (C), the body weight was significantly reduced in the Tx group (C, 4.45 ± 0.28 kg; Tx, 2.70 ± 0.17 kg; P < 0.01) as were the weights of the brain (C, 48.57 ± 1.24 gm; Tx, 42.89 ± 1.54 gm; P < 0.01), lungs (C, 85.39 ± 4.75 gm; Tx, 64.81 ± 6.45 gm; P < 0.05), heart (C, 28.53 ± 1.50 gm; Tx, 17.50 ± 0.97 gm; P < 0.01), and adrenal glands (C, 0.54 ± 0.07 gm; Tx, 0.33 ± 0.06 gm; P < 0.05). Heart rate (C, 158 ± 4.33; Tx, 126 ± 3.67; P < 0.01), blood pressure (C, 46.50 ± 1.44 mm Hg; Tx, 32.20 ± 4.01 mm Hg; P < 0.05), and cephalic blood flow (C, 108.75 ± 2.36 ml/100 gm/min; Tx, 74.50 ± 6.69 ml/100 gm/min; P < 0.01) were significantly reduced in the Tx group. Cerebral O2consumption was not significantly different in the two groups. By the methodology used, no morphologic or histologic differences were noted between the brains of Tx fetuses and those of the C group. Caudate nucleus dopamine levels were significantly lower in the Tx group (C, 29.46 ± 3.01 μg/gm tissue; Tx, 15.85 ± 0.52 μg/gm tissue; P < 0.01). Brain stem/auditory-evoked responses as an index of maturation of the auditory pathway along the brain stem were tested in the Tx and C groups, as well as in a group of adult ewes and newborn and 1-week-old lambs. The latency period to the wave IV–V complex, as well as wave I to IV–V interval representative of central conduction, was found to be significantly prolonged in both the Tx and C groups as compared with adults and newborn and 1-week-old lambs. The values in these latter three groups were not significantly different from each other, and the Tx group did not differ significantly from the C group. Amplitude was lower in the fetal group, and the morphologic features of waveforms were less defined, but more so in the Tx group, perhaps indicative of relative immaturity. Further investigation is needed before definite conclusions can be drawn about the significance of these observations.