What’s the Real Risk for Thyroid Autoimmunity in Pregnancy?

TOPLINE:

In spontaneously pregnant women with initial thyroid-stimulating hormone (TSH) 2.5 mU/L was infrequent and never rose above 4 mU/L. Higher baseline TSH and anti-thyroperoxidase (TPOAb) positivity were associated with progression.

METHODOLOGY:

• Analysis of demographic and biological data from two previously published cohorts (274 women from the Copenhagen cohort [August 1996-May 1997] and 55 women from the Brussels cohort [January 2013-December 2014]) with at least two measurements of TSH and free thyroxine and at least one TPOAb during spontaneously achieved singleton pregnancies.
• All in the Brussels group were TPOAb+, as were 6.9% (n = 19) in the Copenhagen group, whereas the other 93.1% (n = 255) from the Copenhagen group were TPOAb−.
• Sequential blood sampling was done once per trimester during pregnancy.

TAKEAWAY:

• The majority of women, 87.2% (89.8% in the TPO- group and 79.4% in the TPO+ group) had serum TSH ≤ 2.5 mU/L and remained stable below ≤ 2.5 mU/L at the first and the second sampling during pregnancy.
• Women who progressed to serum TSH > 2.5 mU/L from ≤ 2.5 mU/L at the first sampling represented 4.2% of the total population (2.7% of those who were TPOAb− vs 9.4% of TPOAb+; P = .02).
• A decrease in TSH 2.5 m/L threshold at both samplings.
• Maternal age, body mass index, multiparity (≥ 2 prior pregnancies), and smoking during pregnancy were not significantly different between women with and without biochemical progression, in either TPOAb+ or TPOAb− groups.
• Among TPOAb+ women, circulating antibody concentrations were similar (P = .67) between women who showed biochemical progression (median TPOAb level, 378 U/mL vs 281 U/mL in those who did not progress).
• Serum TSH levels at the first sampling were significantly higher in women who showed biochemical progression than those who did not; this was observed in TPOAb− (median TSH, 2.40 mU/L in women with progression vs 1.25 mU/L in women without; P = .01).

IN PRACTICE:

“Our data seems in favour of declining from systematic high frequency follow-up of TSH during pregnancy in these women, which would also be in line with the absence of a beneficial impact of [levothyroxine] on pregnancy outcomes in TPOAb+ euthyroid women,” the authors wrote.

“Future prospective studies in larger cohorts with longitudinal assessment of thyroid function could help to identify factors such as a relevant baseline TSH cutoff, TPOAb cutoff, parity, [human chorionic gonadotrophin] levels, others, for biochemical progression and potential association of this progression with pregnancy outcomes. This could help to better target women that would benefit the most from repeated thyroid function testing during pregnancy and at which interval,” they added.

SOURCE:

Conducted by Aglaia Kyrilli, MD, PhD, of the Department of Endocrinology, Hôpital Universitaire de Bruxelles – Hôpital Erasme, Université Libre de Bruxelles, Brussels, Belgium, and colleagues, this study was published online on July 1, 2024, in Endocrine Connections.

LIMITATIONS:

Underpowered due to limited numbers of subjects and biochemical progression events. A possible selection bias was that only TPOAb+ women in the Brussels cohort had a second blood sampling during pregnancy, consistent with clinical practice guidelines, so progression of TSH in TPOAb− women of this cohort could not be studied.

DISCLOSURES:

Aglaia Kyrilli has made no disclosures. Three other authors’ research salaries were supported by grants from the Kirsten and Freddy Johansen’s Fund, the Novo Nordisk Foundation, and the Danish Research Council, respectively.