Endocrine Abstracts

Introduction: The newly proposed RCOG guidelines suggests dose increment of thyroxine based on prenatal thyroxine doses. The aim of our retrospective audit was to assess if the aetiology of hypothyroidism (and dose of thyroxine) prenatally could help predict the need for dose increment during pregnancy.

Methods: n=100. Local guidelines advised maintaining TSH <2.5 throughout pregnancy. Outcomes of thyroxine adjustment at the first and last visit (in third trimester) reviewed. Data was analysed based on

• aetiology of hypothyroidism: primary (including hemithyroidectomy) vs iatrogenic (radioactive iodine or post total thyroidectomy).

• prenatal dose of thyroxine ≤ 100mg vs >100mg

Results: Among the iatrogenic group, all 4 patients who did not require a dose change had total thyroidectomy for thyroid cancer and were on supraphysiological replacement of thyroxine. Among patients on >100mcg, 11 of the 12 iatrogenic patients required dose increment, which is significantly higher than the primary group.

Conclusion: Dose increment needed during first trimester can be quite variable. Patients with post radioactive iodine or total thyroidectomy induced hypothyroidism represent a significant sub-cohort where dose increment is invariably required, with their higher prenatal dose being a marker of lack of endogenous thyroid activity.