Overt hypothyroidism occurs when the TSH level is increased and the free thyroxine level (FT4) is low. This can cause severe symptoms and is associated with an increased incidence of infertility, miscarriage and other adverse outcomes in those women who are trying to conceive or those who are already pregnant. It is clear that overt hypothyroidism should be treated with thyroid hormone replacement- usually levothyroxine. Subclinical hypothyroidism occurs when the TSH level is increased but the FT4 level remains within the normal range. This is a milder form of the hypothyroidism and may not need to be treated in the absence of pregnancy. Controversy exists whether the subtle abnormalities of thyroid function in subclinical hypothyroidism are truly associated with infertility and miscarriage and whether treatment with thyroid hormone reduces these events.
In September 2015, American Society for Reproductive Medicine Published a guideline on Subclinical hypothyroidism in the infertile female population: a guideline
Following are the summary of findings and major recommendations :-
- Subclinical hypothyroidism is defined as a thyroid-stimulating hormone (TSH) level greater than the upper limit of normal range (4.5–5.0 mIU/L) with normal free thyroxine (FT4) levels.
- The normal reference range for TSH changes in pregnancy. The upper limit of normal in most laboratories is 4 mIU/L for nonpregnant women and 2.5 mIU/L in the first trimester of pregnancy.
- This guideline was conducted because it is controversial whether or not to use first-trimester pregnancy thresholds for upper limit of TSH (i.e., >2.5 mIU/L) to diagnose and treat subclinical hypothyroidism (SCH) in women with infertility who are attempting pregnancy.
- There is insufficient evidence that SCH (defined as TSH >2.5 mIU/L with a normal FT4) is associated with infertility.
- There is fair evidence that SCH, defined as TSH levels >4 mIU/L, is associated with miscarriage, but insufficient evidence that TSH levels 2.5–4 mIU/L are associated with miscarriage.
- There is fair evidence that treatment of SCH when TSH levels are >4 mIU/L is associated with improved pregnancy rates and decreased miscarriage rates.
- There is fair evidence that SCH when TSH levels are >4 mIU/L during pregnancy is associated with adverse developmental outcomes; however, treatment did not improve developmental outcomes in the only randomized trial.
- There is fair evidence that thyroid autoimmunity is associated with miscarriage and fair evidence that it is associated with infertility. Levothyroxine treatment may improve pregnancy outcomes in women with positive thyroid antibodies, especially if the TSH level is over 2.5 mIU/L.
- There is good evidence against recommending universal screening of thyroid function during pregnancy.
- Currently available data support that it is reasonable to test TSH in infertile women attempting pregnancy. If TSH concentrations are over the nonpregnant lab reference range (typically >4 mIU/L), patients should be treated with levothyroxine to maintain levels below 2.5 mIU/L. (Grade B)
- Given the limited data, if TSH levels prior to pregnancy are between 2.5 and 4 mIU/L, management options include either monitoring levels and treating when TSH >4 mIU/L, or treating with levothyroxine to maintain TSH <2.5 mIU/L. (Grade C)
- During the first trimester of pregnancy it is advisable to treat when the TSH is >2.5 mIU/L. (Grade B)
- While thyroid antibody testing is not routinely recommended, one might consider testing anti-thyroperoxidase (TPO) antibodies for repeated TSH values >2.5 mIU/L or when other risk factors for thyroid disease are present. (Grade C)
- If anti-TPO antibodies are detected, TSH levels should be checked and treatment should be considered if the TSH level is over 2.5 mIU/L. (Grade B)
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