Maternal thyroidectomy

The effects of severe iodine deficiency on fetal brain and somatic development were found to be more severe but similar to those observed by McIntosh et al (10) when thyroidectomy was performed on fetal sheep at 50-60 days. Fetal thyroidectomy later in gestation at 98 days caused less severe effects (11). Further investigations by McIntosh et al (12), following earlier indications (13) that the maternal thyroid could exert an influence on fetal development in early pregnancy, revealed that a combination of maternal thyroidectomy before conception and fetal thyroidectomy at 98 days gestation produced more severe effects than those of iodine deficiency (Fig. 1). 

This is further indicated by evidence of the relation of maternal thyroxine levels to the risk of cretinism and the psychomotor defect from studies in Papua New Guinea (26). The subsequent lack of fetal thyroid hormones due to the inadequate supply of iodine in iodine deficiency would exacerbate the effects of maternal thyroid insufficiency and the combination of effects, which were represented experimentally in the sheep by maternal thyroidectomy before conception (21) combined with fetal thyroidectomy at 98 days (12), might be expected to produce the multiple defects of neurological cretinism. 

Studies of the mechanisms involved have been carried out in the sheep. The findings reveal significant, though less severe, effects of fetal thyroidectomy (late gestation) and a significant effect of maternal thyroidectomy on brain development in mid gestation. A combination of maternal and fetal thyroidectomy has similar but more severe effects than iodine deficiency. 

Chavadej, J., Hua, C.H., Cragg, B.G. and Hetzel, B.S. The effect of maternal thyroidectomy prior to conception on fetal brain development in sheep. Acta Endocrinologica 112 93-99 (1986). 

G. Morreale de Escobar, F. Escobar del Rey, R. Pastor, J. Mallol and M.J. Obregon, Effects of maternal thyroidectomy or iodine deficiency on T4 and T3 concentrations in rat concepta, before and after onset of foetal thyroid function, iji lodine nutrition, thyroxine and brain development", N. Kochupillai, M.G. Karmakar and V. Ramalingaswami eds., Tata McGraw-Hill Publ., New Dehli (1986), pp. 109-117. 

To investigate these various possibilities an animal model was developed in the sheep (Figure 62.4), because of the access provided for both maternal and fetal thyroidectomy and previous experience with trace element deficiencies (Hetzel, 1976). Subsequently, a similar model was developed in the primate marmoset monkey Callithrix jacchus jacchus) (Mano et ai, 1987) and in the rat (Li et al., 1985 Morreale de Escobar et ai, 1993). 

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