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Large variation in TSH reference limits seen during first trimester of pregnancy
During the first trimester of pregnancy, thyroid-stimulating hormone reference limits differ widely, according to recent findings.
Frequent misclassification and possible incorrect choice of therapy may be caused by a using a uniform set of reference limits, the researchers wrote.
Peter Laurberg, MD, professor in the department of endocrinology at Aalborg University Hospital in Denmark, and colleagues evaluated data from the Danish National Birth Cohort on 6,671 healthy women who were pregnant between 1996 and 2002 and had TSH and free thyroxin measurements collected during weeks 5 to 19 of pregnancy. Researchers sought to determine week-to-week changes in and predictors of TSH and free T4 reference limits during the first trimester.
Peter Laurberg
Through week 6, TSH and T4 reference levels were the same for pregnant and non-pregnant women. Compared with levels at 5 to 6 weeks, median serum TSH gradually decreased through pregnancy weeks 7 and 8 and were lower in weeks 9 to 12 and then increased again; median serum free T4 levels increased and then decreased again, but these variations were less pronounced than those for TSH levels. During weeks 9 to 12, the 97.5th percentile for TSH was approximately 0.4 mU/L lower lower than the non-pregnancy reference and the 2.5th percentile was at approximately 0.1 mU/L. The 97.5th percentile for free T4 increased by 4.1% and the 2.5th percentile increased by 3.3% from the levels at 5 to 6 weeks.
“Thyroid hormones are essential developmental factors, and from a thyroid disease perspective, early pregnancy is a very important period,” Laurberg told Endocrine Today. “One detail to consider is that the newly formed utero-placenta unit to a considerable degree takes over control of the thyroid from the hypothalamus/pituitary. This leads to week-to-week variations in thyroid function tests, which may complicate the diagnosing and control of thyroid disease in early pregnancy. Clinicians should be aware of the week of gestation when evaluating results of thyroid function tests.” – by Amber Cox
For more information:
Peter Laurberg, MD, can be reached at Department of Endocrinology, Aalborg University Hospital, Sdr. Skovvej 15, 9000 Aalborg, Denmark; email: peter.laurberg@rn.dk.
Disclosure: The researchers report no relevant financial disclosures.