Thyroid function tests in pregnancy

Thyroid function tests in pregnancy

In the first trimester, thyroid stimulating hormone (TSH) is often lower than in the non-pregnant state. This is mostly because of the TSH-like effects of beta-HCG. In the second and third trimester, as beta-HCG decreases, TSH is generally closer to non-pregnant levels. Free thyroid hormones (FT3 and FT4) are often lower in pregnancy, owing to changes in circulating binding proteins.

Different assays can give slightly different results, and therefore may have different reference intervals from one another in both pregnant and non-pregnant individuals.

Reference intervals for TSH in pregnancy

Reference intervals for TSH in pregnant patients measured by the Siemens Centaur platform used at Western Diagnostic Pathology are:

Trimester

Lower reference limit

Upper reference limit

1st

0.05 mU/L

3.5 mU/L

2nd

0.40 mU/L

4.0 mU/L

3rd

0.40 mU/L

4.0 mU/L

Increased TSH in pregnancy

The approach to subclinical hypothyroidism in pregnancy differs between various guidelines and local clinician preferences. This is due to the incompleteness of clinical trial evidence to support thyroid hormone replacement improving obstetric and neonatal outcomes in this setting. Guidelines also differ in the weighting placed upon TPO antibody status to guide treatment decisions.

In general, with the above caveats:

TSH

Recommendation

≥ 10 mU/L

Thyroid hormone replacement is indicated, irrespective of TPO antibodies or FT4

URL - 10 mU/L

Thyroid hormone replacement is generally indicated. TPO antibody status may help to inform this decision, and the need for thyroid hormone replacement post-partum

2.5 mU/L - URL

Thyroid hormone replacement is usually not indicated, although it may be considered in women with positive TPO antibodies

< 2.5 mU/L

Thyroid hormone replacement is not indicated, irrespective of TPO antibodies

URL: Pregnancy-specific upper reference limit, as defined above.

Established hypothyroidism in pregnancy

For women with established hypothyroidism on thyroid hormone replacement, requirements can increase during pregnancy. The target TSH in most cases is around 2 mU/L.

Decreased TSH in pregnancy

Hyperthyroidism in pregnancy is associated with adverse obstetric and neonatal outcomes. Women with TSH below the trimester-specific lower reference limit should have repeat TSH measurement with FT4 and FT3, to distinguish overt hyperthyroidism (in which FT4 and/or FT3 are increased, and TSH is usually below the reporting limit) from subclinical hyperthyroidism (in which FT4 and FT3 are within their respective reference intervals). Measurement of thyroid stimulating immunoglobulin (TSI; also called TRAB) is important to distinguish Graves’ disease from other causes. All pregnant women with overt hyperthyroidism should be reviewed by an endocrinologist, obstetric physician or related specialist.

If further information on thyroid function tests in pregnancy is required, doctors are welcome to contact the Chemical Pathologist at Western Diagnostic Pathology on 08 9317 0999.

References and further reading

Hamblin PS, Sheehan PM, Allan C et al. Subclinical hypothyroidism during pregnancy: the Melbourne public hospitals consensus. Internal Medicine Journal 2019; 49: 994-1000. Available from (abstract only): https://onlinelibrary.wiley.com/doi/pdf/10.1111/imj.14210

Alexander EK, Pearce EN, Brent GA et al. 2017 Guidelines of the American Thyroid Asssociation for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid 2017; 27: 315-389. Available from: https://www.liebertpub.com/doi/pdfplus/10.1089/thy.2016.0457

Smith A, Eccles-Smith J, D’Emden M, Lust K. Thyroid disorders in pregnancy and postpartum. Australian Prescribe 2017; 40: 214-219. Available from: https://www.nps.org.au/australian-prescriber/articles/thyroid-disorders-in-pregnancy-and-postpartum

Medici M, Korevaar TI, Visser WE et al. Thyroid function in pregnancy: what is normal? Clinical Chemistry 2015; 61: 704-713. Available from: http://clinchem.aaccjnls.org/content/61/5/704