Women with pre-conception hypothyroidism often require increased levothyroxine once they are pregnant
Increasing levothyroxine by two extra dosages per week after pregnancy is confirmed prevents development of overt hypothyroidism among most women.1 Women with a pre-conception thyroid-stimulating hormone (TSH) level of < 1.2 mIU/L usually do not require an increase in levothyroxine once they are pregnant.2 Therefore, we suggest maintaining the pre-conception levothyroxine dose when the pre-conception TSH is < 1.2 mIU/L, with adjustments if indicated by TSH monitoring.
Women should avoid ingesting levothyroxine with iron- or calcium-containing supplements
Both calcium and iron supplements have been shown to interfere with absorption of thyroid hormone.3 Ingestion of iron, calcium or prenatal vitamins should be separated from levothyroxine by at least four hours.3
Rates of conception and live birth are not likely influenced by minor thyroid dysfunction4
Thus, women with subclinical hypothyroidism (Box 1) or positive thyroid antibodies (thyroid peroxidase antibody, thyroglobulin antibody) who are planning pregnancy can be reassured.4 Furthermore, a randomized trial of levothyroxine therapy in women with thyroid peroxidase antibodies did not show a difference in spontaneous abortion or preterm delivery rates.5
Box 1: Definitions for thyroid disorders1.
| Disorder | Description |
|---|---|
| Overt hypothyroidism | TSH above the upper limit of normal with low free T4 or TSH > 10 mIU/L |
| Subclinical hypothyroidism | TSH above the upper limit of normal but < 10 mIU/L, with normal free T4 |
Note: TSH = thyroid-stimulating hormone.
Women with active Graves disease or toxic adenoma should delay pregnancy until normal thyroid function is established
The maternal and fetal risk and benefits of available therapeutic options (antithyroid drugs, radioactive iodine, thyroidectomy) should be discussed if pregnancy is planned. Women who receive radioactive iodine should allow for clearance of radioactivity before conceiving (6 months).1
Propylthiouracil is preferred for antithyroid drug treatment before conception
Although a recent study suggests propylthiouracil may be associated with a small increase in congenital malformations,6 it is recommended for women requiring treatment for hyperthyroidism who decline radioactive iodine or thyroidectomy.1 Embryopathy secondary to methimazole use in the first trimester is well established across multiple studies.1
Acknowledgements
The authors would like to thank Sana Ghaznavi MD, Margaret Morris MD and Emmanuel Schaub MD for review of this manuscript.
Footnotes
Competing interests: None declared.
This article has been peer reviewed.
References
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