The background of the study. Women with hypothyroidism may need higher doses of thyroxine (T4) during pregnancy. However, when the need starts to increase and when it is greatest have not been well defined. In this study women with hypothyroidism who were taking T4 were studied prospectively from before conception to the end of pregnancy.
How the study was done. The study involved 19 women who had hypothyroidism, were taking T4, and were planning a pregnancy. Serum T4, thyroid hormone-binding ratio (THBR), free T4 index, and thyrotropin (TSH) were measured before pregnancy, as soon as possible after conception, at two-week intervals for three months, and then monthly until delivery. Their T4 doses were raised as needed to maintain normal serum TSH values (or low values in the six women who had a history of thyroid carcinoma).
The results of the study. Among the 19 women, one had two pregnancies and three became pregnant with the aid of assisted reproduction techniques. Seventeen of the pregnancies ended at term, and three were miscarried. During the pregnancies, serum binding of T4, as measured by the THBR test, increased and remained high. Serum TSH concentrations increased initially and then returned to base line as the doses of T4 were increased, whereas serum free T4 index values did not change.
Serum TSH values exceeded the threshold values calling for an increase in dose of T4 in 16 women (85 percent); they included all 6 women with thyroid carcinoma (average dose increase, 48 percent) and 10 of the other 13 women (average increase, 38 percent). The need for more T4 was first detected between 4 and 16 weeks of gestation. Overall, the increase in dose was greatest between 6 and 16 weeks. As compared with before pregnancy, the mean T4 dose was 30 percent higher at 10 weeks and 50 percent higher at 20 weeks, and it remained at that level thereafter.
The conclusions of the study. Women who have hypothyroidism may need higher doses of T4 during pregnancy. This can occur as early, and, overall, the dose may need to be raised by as much as 85 percent.
The original article. Alexander EK, Marqusee E, Lawrence J, Jarolim P, Fischer GA, Larsen PR. Timing and magnitude of increases in levothyroxine requirements during pregnancy in women with hypothyroidism. N Engl J Med 2004;351:241-9.