Assessment of thyroid function in pregnancy

Editor, – Some further points on testing thyroid function need to be added to the useful information in Associate Professor Tran's review, 'Biochemical tests in pregnancy' (Aust Prescr 2005;28:98-101). First, a small but significant decrease in the concentration of serum free T4, most marked in the third trimester, has been clearly documented.^1,2 In addition, albumin-dependent methods of free T4 estimation show marked negative bias, relative to the non-pregnant reference interval; in the late third trimester, such methods may give subnormal free T4 estimates in up to 50% of samples.³

These methods are unsuitable for assessing thyroid status during pregnancy⁴, unless results are evaluated in relation to reference intervals that reflect method-specific bias at various stages of pregnancy. Clinical chemists need to be aware of this issue when choosing an appropriate free T4 method for obstetric practice and by indicating appropriate reference intervals.

Professor Tran's counsel that 'Graves' disease needs to be rigorously controlled' in pregnancy goes beyond interpretation of test results. This advice must be tempered by the fact that any degree of maternal hypothyroidism in the first trimester can have an adverse effect on fetal brain development^5,6, and that over treatment in the third trimester can be associated with fetal goitre.⁶ As thyrotoxicosis of immune origin often becomes less severe during pregnancy, it is often advisable to decrease the dose of antithyroid drug to minimise the chance of these adverse effects.6 As pointed out by Professor Tran, the exact cause of newly-diagnosed thyrotoxicosis can be difficult to establish in early pregnancy. When the disorder is mild, as judged by clinical rather than laboratory criteria, it may be best followed without treatment for several months until there is a clear indication for active treatment.⁶

Jim R. Stockigt
Epworth and Alfred Hospitals
Professor of Medicine, Monash University
Melbourne

Associate Professor H.A. Tran, author of the article, comments:

Professor Stockigt's comments are appreciated. As usual, they are incisive and informative. The small but significant decrease in serum free tetra-iodothyronine (fT4) can, in part, be explained by the peak of thyroid binding globulin concentrations in the third trimester, although these remain within the reference range in most cases.⁷

Selecting a special method for the obstetric population serviced by the relevant laboratory would always be a challenging task given the large scope of services imposed upon large laboratories by the current practice of pathology. The nuances of such a task are probably best reserved within the realm of clinical biochemists' practice.

As emphasised, the management of thyrotoxicosis in pregnancy is not a simple task. It should not be simply a matter of medication adjustment according to biochemical results, which are never error proof. The literature is littered with, sometimes fatal, adverse reactions⁸where laboratory results as given, are acted upon, when instead a considered and competent clinical assessment is warranted. As inferred by Professor Stockigt, it is best to first do no harm; a caveat that is not applicable to pregnancy alone.