In a US study of more than 17,000 people, overt hypothyroidism and hyperthyroidism were detected in about 4.6% and 1.3% of adults, respectively.1 In this population-based study, thyroid disease was 5 times more prevalent among women than among men. In our ObGyn practices, there are many women of reproductive age with thyroid disease who are considering pregnancy. Treatment of active hyperthyroidism in a woman planning pregnancy is complex and best managed by endocrinologists. Treatment of hypothyroidism is more straightforward, however, and typically managed by internists, family medicine clinicians, and obstetrician-gynecologists.
Clinical management of hypothyroidism and pregnancy
Pregnancy results in a doubling of thyroxine-binding globulin (TBG) levels and a 40% increase in plasma volume, resulting in a need for more thyroxine production.2 Of note, from conception to approximately 13 weeks’ gestation, the sole source of embryonic and fetal thyroid hormones is from the mother.2 Women who have been taking chronic thyroxine treatment may have suppressed thyroid gland activity and be unable to increase thyroxine production in response to pregnancy, necessitating a 30% to 50% increase in their thyroxine dose to maintain TSH levels in the normal range.
For hypothyroid women on long-term thyroxine treatment, recommend increasing the thyroxine dose when pregnancy is recognized. For your patients on chronic thyroxine treatment who are planning a pregnancy, a multiprong approach is helpful in preparing the patient for the increased thyroxine requirements of early pregnancy. First, it is important to counsel the woman that she should not stop the thyroxine medication because it may adversely affect the pregnancy. In my experience, most cases of overt hypothyroidism during pregnancy occur because the patient stopped taking her thyroxine therapy. Second, for hypothyroid women who are considering conception it is reasonable to adjust the thyroxine dose to keep the TSH concentration in the lower range of normal (0.5 to 2.5 mU/L). This will give the woman a “buffer,” reducing the risk that in early pregnancy she and her fetus will have a thyroxine deficit. Third, in early pregnancy, following detection of a positive pregnancy test, your patient can start to increase her thyroxine dose by about two tablets weekly (a 28% increase in the dose). Fourth, TSH levels can be measured every 4 weeks during the first trimester, with appropriate adjustment of the thyroxine dose to keep the TSH concentration below the trimester-specific upper limit of normal (< 4 mU/L).2
TSH and free thyroxine measurements identify women with overt hypothyroidism who need thyroxine treatment. Overt hypothyroidism is associated with adverse reproductive outcomes, including decreased fertility, increased spontaneous abortion, increased fetal loss, and preterm birth.2,3 Hence it is important to immediately initiate thyroxine treatment in pregnant women who have overt hypothyroidism. A diagnosis of overt hypothyroidism is indicated in women with an intact hypothalamic-pituitary axis and a TSH level ≥10 mU/L plus a low free thyroxine concentration. A TSH level of >4 to 10 mU/L, with normal free thyroxine concentration, is evidence of subclinical hypothyroidism (SCH). Among women, there are about 5 times more cases of SCH than overt hypothyroidism.
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