Screening for thyroid disease during pregnancy

Q. A 40-year-old G1 P0 at 8 weeks' gestation with a singleton pregnancy presents to your practice for her first prenatal visit after successful in vitro fertilization. She is otherwise healthy and has no complaints. She read on the Internet that thyroid function testing is being done by many physicians and wants to know if you recommend it. Should thyroid function screening be done routinely in gestation?

What expert guidance exists on screening for thyroid disease in pregnancy?

What are the indications for thyroid function testing in gestation?

What are values for thyroid function in pregnant women?

Gestational age-specific normative ranges for TSH have been described, but in routine practice, most clinicians use laboratory-specific values for nonpregnant adults that most often include an upper range of 2.5 mIU/L to 4.0 mIU/L.² A threshold of 4.0 mIU/L is the cutoff in an ongoing study of subclinical hypothyroidism in pregnancy sponsored by the National Institute of Child Health and Human Development (NICHD).⁶ The 3 components of subclinical hypothyroidism are serum TSH 2.5 mIU/L to 10mIU/L, fT₄ concentration 0.7 ng/dL to 1.8 ng/dL, and absence of symptoms.²