Hypothyroidism is defined as a low FT4 levels with high TSH. The main cause is iodine deficiency, in areas where iodine intake is sufficient the most frequent cause is autoimmune thyroiditis. Other causes are prior thyroidectomy, radioiodine therapy, the use of drugs, congenital hypothyroidism, pituitary or hypothalamic disease, and immunoglobulin binding to the TSH receptor, blocking its activity. Hypothyroidism occurs in 2.5% of pregnancies (19
); however, the frequency of OH is estimated to be between 0.2 and 1.0% (21
Symptoms of hypothyroidism can often be masked by the hypermetabolic state of pregnancy.
In a retrospective study, Haddow et al. in 1999 first described reduced intelligence quotient (IQ) in babies born from hypothyroid mothers corroborating the association between hypothyroidism and increase risk of impaired neurodevelopment in the offspring (23
The authors selected from 25,216 pregnant women 62 with serum TSH values above the 98th percentile in combination with low FT4 values and 124 matched women with normal values. Of these 62 women with thyroid deficiency, 48 were not treated for the condition during the pregnancy.
Their 7–9 year old children, none of whom had hypothyroidism as newborns, underwent 15 tests relating to intelligence, attention, language, reading ability, school performance, and visual-motor performance (Wechsler Intelligence Scale for Children, 3rd edition). The children of the 62 hypothyroid women revealed an IQ averaged 4 points lower than those of the children of the 124 matched control women (P = 0.06, not significant). Furthermore, 15% of the children of the affected mothers had IQ scores of 85 or less, compared with 5% of the matched control children born to euthyroid mothers. The full-scale IQ scores of their children averaged 7 points, significantly lower than those of the 124 matched control children (P = 0.005). The authors conclude that “undiagnosed hypothyroidism in pregnant women may adversely affect their fetuses; therefore screening for thyroid deficiency during pregnancy is warranted”.
Several important obstetrical complications are the increased risk of spontaneous miscarriage, stillbirth and perinatal death. Other frequent complications are preterm delivery, fetal distress and increase in frequency of low birth weight infants (24
), while the occurrence of gestational hypertension, placental abruption and postpartum hemorrhage have been shown to be increased in some, but not all, studies (27
Abalovich et al. in 2002 demonstrated the fundamental importance of Levothyroxine (LT4) treatment to prevent fetal loss (incidence of 4% in adequate substitutive treated group versus 31% in inadequately treated group) (28
Tan et al. in 2006 reported no increase in obstetrical and neonatal complications in treated hypothyroid women (29
These results are validated by Negro et al. in 2010; they assessed adverse events in women with subclinical and overt hypothyroidism and saw that untreated thyroid dysfunction patients had a significantly higher rate of complications compared with those receiving treatment (30
LT4 is the drug of choice for the treatment of hypothyroidism. In women affected before pregnancy, it’s recommended to adjust the dose in order to have a pre-pregnancy TSH lower than 2.5 mIU/L and to maintain the same TSH level during the first trimester, and not exceed 3.0 mIU/L during the second and third trimester (31
The LT4 dosage should be increased at beginning of pregnancy by 30–50%, and may be further increased during the second or the third trimester (32
If hypothyroidism is diagnosed during pregnancy, it’s crucial to restore euthyroidism as soon as possible (34