Monitoring Levothyroxine Dose during Pregnancy: A Prospective Study

Problem statement: Thyroid dysfunction in pregnant women can influence the outcome for mother and fetus at all stages of pregnancy. As the fetus is entirely dependent on maternal thyroid hormones for its development until about 13 weeks of gestation, it is important to ensure adequate thyroxine substitution in pregnant women during the first trimester. The aim of this prospective study was to explore whether hypothyroidic pregnant women are adequately levothyroxine (L-T₄) substituted in early pregnancy. Approach: During March 2008 to July 2009, 93 pregnant females with thyroid diseases were followed at the outpatient department of INMAS. At the first visit 86 patients were on L-T₄ substitution for hypothyroidism. Seven other patients had hyperthyroidism. The patients were regularly followed every 4-8 weeks during pregnancy for dose adjustment. Before each visit serum Free Thyroxine (FT₄) and TSH concentrations were determined. Results: Of the 86 patients on thyroxine substitution for hypothyroidism 56 (65.12%) had serum TSH values within the reference range at their first TSH test. Thirty (34.9%) had TSH values outside the reference range. In 5 patients TSH was <0.27 μIU mL^-1. Fifty (58.13%) of the patients had to increase their thyroxine dose during pregnancy. The initial L-T₄ increase at the first evaluation during pregnancy was 17.46±30.8 μg day^-1. In the 50 patients who needed to increase L-T₄, 26% reached a definitive therapeutic dosage within 12th week of pregnancy, 24% within the 20th week and 50% within the 31st week. Conclusion/Recommendations: In 34.9% of pregnant women on L-T₄ substitution for hypothyroidism, serum TSH values were abnormal when first tested and they had increased chances of fetal loss if not treated timely. Thyroid function in pregnant women on thyroxine substitution should be monitored as soon as pregnancy has been confirmed and carefully followed during pregnancy.