Our statement (11 September 1970, p. 76) that in pregnancy hypothyroid patients probably should be switched from thyroxine to liothyronine has been cogently disputed. An important point in favour of letting the patient continue on thyroxine is that this has given satisfactory results. One correspondent has followed 10 athyroidal patients through their pregnancies. All were maintained on an unchanged dose of thyroxine, all showed the expected rise in PBI, and all gave birth to normal infants. Another point is that in the latter part of pregnancy the foetus produces enough thyroid hormone for its needs; it is not clear whether the risk of foetal hypothyroidism is directly influenced by the concentrations of thyroxine and liothyronine in the maternal blood - little of either passes the placenta. Furthermore, both are present in patients taking thyroxine, since, as we stated (p. 75) about one-third of administered thyroxine is metabolised to liothyronine.
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