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There is considerable uncertainty about the management of hypothyroidism in pregnancy. Our aim was to establish the pattern of thyroxine dose adjustment needed and to determine the clinical reasons for these changes and the contributory factors. Of 89 pregnancies, thyroxine dose was unchanged in 50, increased (by a mean of 38 micrograms) in 34, and decreased in 5. Twenty-three percent of women who were tested in the first trimester needed an immediate increase in thyroxine. One-quarter (26%) of the women who needed a gestational increase in thyroxine dose had had a recent pre-pregnancy increase in thyroxine requirement (compared with 0% in women on static dose in pregnancy, P < 0.001). Furthermore, they did not require a decrease in thyroxine dose postpartum, suggesting a long-term need for more thyroxine rather than a transient gestational effect. None of the women who had stable doses of thyroxine during pregnancy had required recent pre-pregnancy changes in dose or needed postnatal changes. Inadequate pre-pregnancy control of thyroid function is associated with a need to increase thyroxine dosage during pregnancy.