Suppression of maternal pituitary thyroid-stimulating hormone during pregnancy

Maternal serum thyroid-stimulating hormone (TSH) has been estimatedin 38 pregnant women with three new specific non-radioactivemonoclonal immunoassays which can measure very low concentrations.Values for the Pharmacia-LKB ‘DELFIA’ fluoroimmunoassayand for the Amersham ‘Amerlite’ luminescence immunoassaywere statistically identical over the range 0.15–1.5 uIU/mlWHO 2nd IRP 80/558, but thereafter the Amersham values wereslightly lower. The Abbott ‘IMx’ assay system gaveslightly higher results commensurate with the quoted highernon-pregnant normal values. The Amersham assay was preferredon practical grounds. There was no relationship of TSH levelsto HCG levels, or to the length of pregnancy. Four women hadapparently zero TSH levels by the Amersham and Pharmacia-LKBassays, although the Abbott assay could detect very low concentrations.Together with women whose TSH levels were below the normal non-pregnantrange for each assay, there were a total of eight women (21%)with TSH levels below ‘normal’. This suppressionof maternal pituitary TSH levels during pregnancy was consideredto be due to the central feedback inhibitory thyrotrophic activityof HCG, as a separate placental thyrotrophic hormone is believednot to exist. The enlargement of the maternal thyroid glandand the increased production of thyroid hormones during normalpregnancy is likewise to be attributed to HCG rather than toan increase in TSH production.