Thyroid Disease During Pregnancy

Options for Management

Tuija Männistö


Expert Rev Endocrinol Metab. 2013;8(6):537-547. 

In This Article

Isolated Hypothyroxinemia

Isolated hypothyroxinemia is characterized by low fT4 concentrations with normal serum TSH levels.[2] Hypothyroxinemia can be due to relative iodine deficiency where the thyroid produces triiodothyronine instead of thyroxine to preserve iodine as raw material, but it is a condition also observed in populations with iodine sufficiency.[64] Notably, by definition of reference intervals, 2.5% of healthy women will also have fT4 concentrations below the lower reference limit.

Isolated hypothyroxinemia was associated with preterm birth, infants weighing more than 4000 g and gestational diabetes in one study,[34] but these associations were not seen in another cohort.[25] Some association has also been seen with neonatal intraventricular hemorrhage, but this association was based on a very limited sample size.[25] Like hypothyroidism, hypothyroxinemia has been associated with poorer neuropsychological development in the offspring.[65,66] However, as it is unclear whether the association between adverse outcomes and hypothyroxinemia are due to iodine deficiency or maternal thyroid disease,[64] levothyroxine treatment for isolated hypothyroxinemia cannot be recommended.[2] Adequacy of iodine nutrition should be ensured in women with isolated hypothyroxinemia.[64]