Classification of subclinical hypothyroidism -Types of SCH

Subclinical Hypothyroidism and Infertility: Who Should Be Treated?

Why a New Guideline?

In recent years, accumulating evidence has suggested that the normal range for thyroid-stimulating hormone (TSH) levels should be lowered during pregnancy and a new category of subclinical hypothyroidism be created. Since then, several reviews have dealt with the problem of TSH levels between 2.5 mIU/L and 4 mIU/L (or the upper normal value for the lab) and fertility and pregnancy outcomes. The recommendations of the American Society for Reproductive Medicine on the management of subclinical hypothyroidism in the infertile female patient.

Guideline Summary

Thyroid dysfunction can be classified as hypo- or hyperthyroidism. Hypothyroidism can be further classified as A) overt hypothyroidism (elevated TSH and low thyroid hormone levels) and B) subclinical hypothyroidism (SCH) (elevated TSH and normal thyroid hormone levels).

Most laboratories use an upper normal value for TSH between 4 mIU/L and 4.5 mIU/L. Some evidence suggests that the upper normal TSH level in pregnancy should be lowered to 2.5 mIU/L in the first trimester, 3 mIU/L in the second trimester, and 3.5 mIU/L in the third trimester.

Overt hypothyroidism is associated with infertility, miscarriage, and adverse pregnancy outcomes, and may result in delayed neurodevelopment in the fetus. Therefore, this condition requires treatment. The associations between SCH and infertility or pregnancy outcome are less obvious, however.

SCH can be further divided into persons with TSH levels above the upper limit of normal and those with TSH levels between 2.5 mIU/L and 4 mIU/L. SCH seems to be more common in infertile women (especially those with unexplained infertility) compared with the general population. Miscarriage rates are higher among women with SCH and a TSH level > 4 mIU/L, but the association is less clear in women with TSH levels between 2.5 mIU/L and 4 mIU/L.

Likewise, placental abruption, preterm delivery, and premature rupture of membranes are more common among women with SCH and TSH levels > 4 mIU/L, but perinatal outcomes of women with TSH levels between 2.5 mIU/L and 4 mIU/L have not yet been studied adequately.

The evidence is at best fair for an association between SCH with TSH levels > 4 mIU/L and neurodevelopmental delay, and there is no evidence for adverse central nervous system effects of SCH with TSH levels between 2.5 mIU/L and 4 mIU/L.

Women with SCH and TSH levels > 4 mIU/L who receive levothyroxine treatment have improved pregnancy rates and perinatal outcomes.

The evidence is insufficient, however, on the impact of levothyroxine treatment on pregnancy rates or perinatal outcomes with TSH levels between 2.5 mIU/L and 4 mIU/L.

·         Preconceptionally diagnosed hypothyroid women (overt or subclinical) should have their T4 dosage adjusted such that the TSH value is less than 2.5 μIU/mL before pregnancy. 

·         .The T4 dosage in women already on replacement will routinely require a dose escalation (30% to 50%) at 4 to 6 weeks gestation in order to maintain a TSH value less than 2.5μIU/mL.
.Pregnant women with overt hypothyroidism should be normalized as rapidly as possible to maintain TSH at less than 2.5 and 3 μIU/mL in the first, second, and third trimesters, respectively.
Euthyroid women with thyroid autoantibodies are at risk of hypothyroidism and should have TSH careening in each trimester.
.After delivery, hypothyroid women need a reduction in T4 dosage used pregnancy. Because subclinical hypothyroidism is associated with adverse outcomes for mother and the fetus, T4 replacement is recommend.