Thyroxenemia in Pregnancy
Based on the provided evidence, "thyroxenemia" appears to be a term referring to hypothyroxinemia (low thyroxine levels) in pregnancy, which represents a condition where maternal free thyroxine (FT4) levels are reduced while TSH remains normal.
Definition and Clinical Significance
Hypothyroxinemia in pregnancy is characterized by low maternal FT4 levels with normal TSH values, representing a distinct entity from overt or subclinical hypothyroidism. 1
Key Features:
- Low free thyroxine (FT4) or free thyroxine index (FTI) 2
- Normal thyroid-stimulating hormone (TSH) levels 1
- Can occur even in women on adequate thyroxine replacement therapy 3
Maternal-Fetal Implications
Fetal Development Concerns:
- Maternal hypothyroxinemia is associated with adverse obstetric outcomes and impaired child neurodevelopmental outcomes 1
- The fetus depends on maternal thyroxine during the first trimester before its own thyroid function develops 4
- Maternal and fetal hypothyroxinemia can lead to irreversible CNS damage 4
- Untreated maternal hypothyroidism may have adverse effects on fetal neurocognitive development 5
Pregnancy Complications:
- Associations with pregnancy loss and preterm labor have been documented 1
- Lower scores on child cognitive assessment are linked to maternal hypothyroxinemia 1
Monitoring and Management Considerations
For Women on Thyroxine Replacement:
- Up to 75% of women with hypothyroidism on thyroxine require higher doses during pregnancy to maintain normal TSH levels 6
- In one study, 49% of pregnant women on thyroxine substitution had TSH values outside the reference range when first tested in early pregnancy 3
- Fetal loss was significantly higher (29% vs 6%) in women with abnormal TSH values compared to those with normal values 3
Testing Recommendations:
- TSH testing is the recommended initial screening test 2
- Both TSH and FT4 or FTI should be performed in pregnant women with suspected thyroid dysfunction 2
- Thyroid function should be monitored early in pregnancy, with TSH assessed once each trimester 6
- Pregnancy may increase levothyroxine requirements, necessitating dose adjustments 5
Treatment Controversy
Although treatment of mild maternal thyroid hypofunction (including isolated hypothyroxinemia) remains controversial due to lack of clinical trials showing improved outcomes with levothyroxine treatment, there is minimal risk of levothyroxine therapy to keep TSH within pregnancy-specific reference ranges. 1
Important Caveats:
- The distinction between hypothyroxinemia and overt hypothyroidism is critical, as management approaches differ 1
- Women with overt hypothyroidism should be treated with levothyroxine to achieve TSH <2.5 mIU/L 1
- Levothyroxine should not be discontinued during pregnancy, and hypothyroidism diagnosed during pregnancy should be promptly treated 5