Assessment of thyroid function in intensive care unit patients by liquid chromatography tandem mass spectrometry methods

Highlights

• Thyroid function was measured in ICU patients by LC-MSMS and compared to immunoassay.

• TT3 and TT4 were significantly lower when measured by LC-MSMS than immunoassay.

• FT4 values were elevated by LC-MSMS (38%) compared to 2% by immunoassay. Reverse T3 was elevated in 62% of patents.

• Discrepancies between reference LC-MSMS and immunoassay have clinical implications.

Abstract

Objectives

Patients with non-thyroidal illness syndrome have many abnormalities in thyroid hormone tests. Such patients have medical comorbidities associated with low serum proteins and are on multiple medications that interfere with thyroid hormone measurement by immunoassay platforms. It is unknown if these thyroid hormone measurements reflect physiologic conditions or if they are artifacts of testing methodology.

Methods

Fifty patients were selected from the intensive care unit (ICU) from our institution. Total and free thyroid hormones in plasma were measured by gold standard liquid chromatography-tandem mass spectrometry (LC-MSMS). The results were compared to the Roche Cobas 6000. Patient medical comorbidities and binding protein levels were assessed.

Results

Concentrations of total 3,5,5′-triidothyronine (TT3) and total thyroxine (TT4) were significantly more likely to be low by LC-MSMS compared to immunoassay. Free 3,5,5′-triidothyronine (FT3) levels were similar by immunoassay and LC-MSMS. However, FT4 concentrations were mildly elevated for many patients when measured by ultrafiltration LC-MSMS (19/50, 38%) compared to 1/50 (2%) when measured by immunoassay (p = 0.0001). Decreased albumin and thyroxine binding globulin were common and patients were on an average of 11.7 ± 5.0 medications, all factors known to interfere with results found on immunoassays.

Conclusions

Marked discrepancies in thyroid hormone measurement were noted between reference LC-MSMS and a common immunoassay platform. It is hypothesized that T4 binding to low affinity albumin is displaced by several drugs, raising concentrations of FT4 by LC-MSMS compared to immunoassay, and that the immunoassay values are falsely decreased due to low binding proteins in our patient population.

Introduction

Hospitalized patients suffer from a number of medical conditions and receive many medications that cause typical changes in thyroid function tests. These changes are referred to as “non-thyroidal illness” or the “euthyroid sick syndrome.” Patients with non-thyroidal illness are reported to have decreased total 3,5,5′-triidothyronine (TT3) and free 3,5,5′-triidothyronine (FT3), increased reverse 3,5,5′-triidothyronine (rT3), and may have normal or decreased total thyroxine (TT4) and free thyroxine (FT4) [1] when measured by immunoassay. Thyroid stimulating hormone (TSH) is usually normal, and thus these patients are considered “euthyroid.” These changes in thyroid status are reported to occur in over 70% of hospitalized patients [1]. The marked abnormalities in thyroid hormone tests that occur in non-thyroidal illness are generally considered to reflect a response to systemic illness rather than clinical thyroid dysfunction, and treatment with thyroid hormone is currently not recommended by most Endocrinology organizations including the American Thyroid Association [2], [3]. Furthermore, routine testing for thyroid function in hospitalized patient in the absence of suspected thyroid disease is not recommended by most guidelines.

Thyroid hormones are measured by automated immunoassay platforms in the majority of clinical laboratories. However, a number of studies have shown discrepancies in free and total thyroid hormone measurement by immunoassay when compared to a reference method such as liquid chromatography-tandem mass spectrometry (LC-MSMS) in several populations (reviewed in [4], [5]). Multiple studies have shown falsely normal values for TT3, FT3, and FT4 by immunoassays that are below the reference interval when measured by reference LC-MSMS in a number of patient populations [3], [6], [7], [8]. Furthermore, several disease and physiologic conditions are known to cause interference in thyroid hormone measurement by immunoassay. Specifically, changes in the thyroid hormone binding proteins thyroxine-binding globulin (TBG) and albumin that occur in a number of conditions cause unreliable immunoassay free thyroid hormone measurements [9], [10]. Many commonly used drugs such as heparin, furosemide, anti-epileptics, and salicylates displace thyroid hormone binding from serum proteins [11], [12], [13]; immunoassays that use a dilution step cause a reduction in competing drugs, resulting in increased in vitro free thyroid hormone binding to proteins and a falsely decreased measure of free thyroid hormone [4]. Hospitalized patients, especially those in intensive care units (ICU), have a number of conditions that cause low protein states and are frequently on a large number of medications known to cause artifactual values in thyroid hormone assays. Thus, it is not clear if the abnormalities seen in non-thyroidal illness reflect physiologic conditions or if they are at least partially artifact due to analytical issues.

To our knowledge, no study has extensively studied thyroid hormone measurement by gold standard, validated LC-MSMS methods in a population of ICU patients. This study evaluates plasma FT3, FT4, TT3, TT4, and rT3 determined by reference method LC-MSMS in a cohort entirely of ICU patients. The results are further compared to a common automated immunoassay platform. Marked discrepancies were found by the immunoassay method in this population with multiple medical comorbidities, low binding proteins, and on multiple medications. These results have important implications for clinical studies evaluating patients with non-thyroidal illness syndrome.