Differentiated thyroid cancer (DTC) is common in female patients of reproductive age and generally has a good prognosis and so many patients may become pregnant following treatment. Progression of DTC during pregnancy has been reported and may relate to thyroid stimulation by human chorionic gonadotropin (hCG). Until recently suppression of thyrotropin (TSH) by supraphysiological doses of levothyroxine was indicated for all patients following thyroidectomy and radioiodine ablation for DTC. We retrospectively analysed 31 patients during pregnancy between 2008 and 2014 following successful treatment of DTC to determine 1) changes in thyroglobulin (TG) levels during pregnancy and 2) the effect of TSH suppression on birth weight.
Thyroglobulin measurements during pregnancy were available in 28/31 patients, of which 26 remained suppressed, defined as TG <0.90 μg/l. One patient had elevated levels prior to pregnancy with no significant increase during follow up (2.35 μg/l to 2.94 μg/l) and one patient had a rise in TG (0.48 μg/l to 1.65 μg/l), with no evidence of recurrence on neck ultrasound and a return to baseline levels following delivery. TSH levels were lower in the third trimester compared to first and second, with 69%, 89% and 97% achieving levels <0.2miu/l respectively in each trimester. The mean free T4 levels were similar in each trimester (19.6±0.6 pmol/l, 18.5±1pmol/l and 19.8±0.4 pmol/l respectively). Delivery weight was available for 20 pregnancies with a mean birth weight of 3.5±0.6 kg, which is similar to the UK national average.
In our cohort of patients there was no evidence of DTC relapse during pregnancy and routine monitoring of TG levels in pregnancy may not be necessary unless pre-conception TG levels are elevated. Suppression of TSH levels did not affect birth weight, and if required during pregnancy, should not adversely affect outcome provided free T4 levels are maintained within the normal reference range.