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Hypothyroid women in 1st trimester need 29% higher levothyroxine dose

Reuters Health • The Doctor's Channel Daily Newscast

NEW YORK (Reuters Health) – When hypothyroid women become pregnant, they need to increase their levothyroxine dose by two tablets per week, or 29%, a new trial has shown.

This protocol “significantly reduces the risk of maternal hypothyroidism during the first trimester and mimics normal physiology,” according to senior author Dr. Erik K. Alexander from Brigham and Women’s Hospital, Boston, and colleagues.

Their findings are from the randomized THERAPY trial of women with treated hypothyroidism who wished to become pregnant. As soon as pregnancy was confirmed, the women increased their weekly levothyroxine dose either to nine tablets (double doses on Saturdays and Wednesdays) or 10 tablets (double doses on Mondays, Wednesdays and Fridays).

Dr. Alexander and his colleagues had previously reported that levothyroxine requirements may rise as early as the fifth week of gestation (see Reuters Health report, July 14, 2004).

The women started the protocol at a median of 5.5 weeks of pregnancy. By that time, 13 (27%) already had TSH values over 5.0 mIU/L, “confirming inadequate T4 replacement in early gestation,” the authors say.

The investigators measured thyroid hormones every 2 weeks until 20 weeks and once more at 30 weeks. They adjusted levothyroxine dosing every 4 weeks to maintain thyroid stimulating hormone (TSH) levels at 0.5 to 4.9 mIU/L for women with benign thyroid disease and to 0.1 to 0.49 mIU/L for women with thyroid cancer. On the intervening weeks, they adjusted the dose only if TSH was higher than 10 or less than 0.1 mIU/L.

In some women, the initial levothyroxine dose augmentation over-suppressed TSH to the point where the dose needed to be reduced; this occurred in 8 women (32%) in the 9-tablet group and 15 women (65%) in the 10-tablet group.

The researchers say the following patients are at particular risk for TSH suppression: athyreotic patients, those with prepregnancy TSH levels < 1.5 mIU/L, and those whose prepregnancy dose was at least 100 mcg/day. These women need closer monitoring or perhaps a more conservative initial levothyroxine adjustment, the authors say.

In general, however, monitoring thyroid function every 4 weeks until 20 weeks and again at 30 weeks detected 92% of abnormal TSH values in this study, the researchers report.

“Our data emphasize the importance of prenatal counseling for treated hypothyroid women,” the authors conclude. “Despite median study enrollment of 5.5 weeks gestation, nearly 30% of women were already hypothyroid.”

“In clinical practice, women do not typically seek obstetrical care before 8-12 weeks gestation,” they add. “Thus, patients themselves must understand the importance of initial (levothyroxine) adjustment immediately upon a missed menstrual cycle and a positive home pregnancy test.”

Reference:
http://jcem.endojournals.org/cgi/content/abstract/jc.2010-0013v1J Clin Endocrinol Metab 2010.