Effects of increased thyroxine dosage pre-conception on thyroid function during early pregnancy.

OBJECTIVE

To compare the effects of pregnancy on the serum free thyroxine (FT4) levels in two cohorts of primary hypothyroid women treated with different levothyroxine (L-T4) doses before gestation.

DESIGN AND METHOD

Twenty-five women with compensated hypothyroidism of different aetiology (thyroidectomized and Hashimoto's thyroiditis) were enrolled in this prospective study. The women were receiving substitutive doses of L-T4 and were anticipating pregnancy. They were assigned to two groups: 14 patients (group I) were switched to partially suppressive treatment while 11 patients (group II) continued the same therapeutic regimen.

RESULTS

Pre-conceptional thyroid function evaluation demonstrated significantly higher FT4 and lower TSH in group I (P<0.001, for both hormones) and comparable free 3,5,3'-triiodothyronine (FT3) levels. The first post-conception thyroid function evaluation occurred at a median time of 6 (5-8) and 7 (5-9) weeks of gestation, for groups I and II respectively (P<0.05); all women in group I showed adequate serum FT4 levels while three patients in group II showed low-normal FT4 levels and one case was below normal levels. Statistical analysis demonstrated significantly higher frequencies (0% vs 36.4%; P<0.05) of low-normal FT4 levels in patients receiving substitutive doses of L-T4. None of the Hashimoto's-affected patients showed low or low-normal serum FT4 levels regardless of their therapeutic regimen.

CONCLUSION

Our results suggest that in hypothyroid women anticipating pregnancy (with serum TSH in the lower quartile of normal range), the pre-conception adjustment of L-T4 doses may result in adequate maternal thyroid function up to the first post-conception evaluation. The procedure seems safe and inexpensive; it may be a worthwhile treatment, at least in thyroidectomized women, in view of the well-known potential effects of even marginal maternal thyroid hypofunction on the subsequent IQ of the progeny.