March 2002 -- Studies published earlier this year found that women with thyroid disease are at more risk than previously thought of having infants who have birth defects, including abnormalities of the heart, kidney or brain, and other defects such as cleft lip and cleft palate. With another perspective on this issue, Argentine researchers studied 150 pregnancies among women ages 16-39 years who were hypothyroid while pregnant. 34% of the women -- 51 pregnancies -- were hypothyroid when they conceived. Among them, 16 were overtly hypothyroid -- TSH more then 33.4 -- and 35 were subclinically hypothyroid -- TSH more than 12.9. A total of 99 pregnancies were conceived when the women were euthyroid -- normal TSH range -- and receiving thyroid treatment.

What the researchers found was that the treatment with levothyroxine was not sufficient, meaning that the women had elevated TSH levels, the outcome of pregnancy was spontaneous abortion (miscarriage) in 60% of those who were overtly hypothyroid patients and in 71.4% of subclinically hypothyroid patients. 20 % of overtly hypothyroid patients and 7.2% of the subclinically hypothyroid had premature delivery. When treatment was sufficient and adequate, 100% of overtly hypothyroid patients and 90.5% of subclinically hypothyroid patients carried the pregnancies to term, with no miscarriages. Miscarriages, premature and term deliveries in patients who were euthyroid on levothyroxine at the time of conception were 4%, 11.1% and 84.9%.

Interestingly, another important finding of the researchers was that among patients taking levothyroxine for hypothyroidism prior to conceiving, Almost 70% had to increase their dose during pregnancy.

Overall, the researchers concluded that the success of a pregnancy doesn't depend on whether the hypothyroidism was overt or subclinical, but primarily on receiving adequate treatment.

(Reference: Abalovich M, et. al. "Overt and subclinical hypothyroidism complicating pregnancy," Thyroid. 2002 Jan;12(1):63-8.)

NOTE: For more information, visit the Thyroid & Pregnancy Information Center.