Prediction of post partum thyroid dysfunction: can it be improved?

BACKGROUND: Screening pregnant women for thyroid peroxidase antibodies (TPOAb) to identify those at risk for post partum thyroid dysfunction (PPTD) is controversial, mainly because of the low positive predictive value (ppv) of TPOAb.

OBJECTIVES: To evaluate if the ppv of TPOAb can be enhanced, either by taking into account the time of TPOAb testing, or by combining this parameter with other putative determinants of PPTD such as smoking, family history or other autoimmune diseases.

METHODS: A prospective study was performed in the Kempenland region (southeastern Netherlands). Three hundred and ten unselected women were visited at 12 and 32 weeks gestation and 4, 12, 20, 28 and 36 weeks post partum. Serial thyroid stimulating hormone (TSH), free thyroxine (fT4) and TPOAb testing was performed. Thyroid dysfunction (TD) was defined as abnormal TSH either in combination with abnormal fT4 (overt TD) or without abnormal fT4 (subclinical TD). PPTD was defined as overt TD post partum. Multivariate regression analysis was performed for determining independent risk factors for PPTD. The sensitivity and specificity of TPOAb at different time points and at different concentrations were calculated and presented in receiver operating characteristic (ROC) curves. Women who had experienced PPTD were followed for 2.5-3 years.

RESULTS: Data from 291 women were available for analysis. Serum fT4 declined during pregnancy and returned to baseline values post partum. TD in gestation was present in 23 women (7.9%): serum TSH was transiently decreased in 13 (6 had overt gestational thyrotoxicosis (2.1%)) and increased in 10 (2 had TPOAb). Both point prevalence and concentration of TPOAb decreased during gestation and returned to baseline levels within 12 weeks post partum. TD in post partum was present in 36 women (12.4%): 21 had subclinical and 15 overt TD. Out of the 15 women with overt TD (incidence of PPTD: 5.2%) 10 were positive for TPOAb (TPOAb+): 9 had thyrotoxicosis (4 TPOAb+), 5 hypothyroidism (5 TPOAb+) and 1 thyrotoxicosis followed by hypothyroidism (TPOAb+). Independent risk factors for PPTD were TPOAb (relative risk (RR) = 2 7.2), bottle feeding (RR = 11.1) and smoking habits (ever smoked: RR = 3.1; women with PPTD had smoked more cigarettes for a longer period of time). The sensitivity of TPOAb testing was highest at 12 weeks gestation (0.67). The ppv of TPOAb was 0.31-0.75 (depending on time of testing and concentration), increasing slightly to 0.38-0.80 when combined with bottle feeding or smoking habits. There appeared to be an autoimmune form of PPTD in 2/3 of cases and a non-autoimmune form; women with the autoimmune form were at risk for developing permanent hypothyroidism.

CONCLUSIONS: A maximum of 2/3 of PPTD cases can be predicted from the presence of TPOAb because 1/3 remained negative for TPOAb. The most appropriate time for TPOAb testing is in the first trimester of pregnancy. The combination of TPOAb testing with anamnestic determinants of PPTD does not increase ppv substantially.