CASE REPORT
Year : 2022 | Volume
: 19 | Issue : 4 | Page : 262--263
Spuriously low thyroid-stimulating hormone? A laboratory phenomenon
S Danalakshmi1, Mamta Soni2,
1 Department of Biochemistry, Apollo Hospitals, Chennai, Tamil Nadu, India
2 Department of Hematology and Clinical Pathology, Apollo Hospitals, Chennai, Tamil Nadu, India
Correspondence Address:
Dr. Mamta Soni
Department of Hematology and Clinical Pathology, Apollo Hospitals, 21 Greams Lane, Off. Greams Road, Chennai - 600 031, Tamil Nadu
India
Abstract
Thyroid gland is an endocrine gland with vital functions regulating metabolism, growth, and development. The tests used to assess the functions of the thyroid gland include thyroid-stimulating hormone (TSH), free thyroxine, and free tri-iodothyronine. TSH is routinely measured in the clinical laboratory with automated immunoassays to diagnose and monitor thyroid disorders. We present a case wherein the discrepancy between the clinical symptoms and the biochemical test results raised the possibility of methodological interference in laboratory testing, finally leading to a detection of a rare genetic mutation in the patient. This case reiterates that clinical correlation of laboratory results and effective communication between the clinician and the laboratory are imperative for a correct diagnosis and therapeutic interventions.
How to cite this article:
Danalakshmi S, Soni M. Spuriously low thyroid-stimulating hormone? A laboratory phenomenon.Apollo Med 2022;19:262-263
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How to cite this URL:
Danalakshmi S, Soni M. Spuriously low thyroid-stimulating hormone? A laboratory phenomenon. Apollo Med [serial online] 2022 [cited 2023 Mar 23 ];19:262-263
Available from: https://apollomedicine.org/text.asp?2022/19/4/262/354226 |
Full Text
Introduction
Thyroid gland is an endocrine gland with vital functions regulating metabolism, growth, and development. The tests used to assess the functions of the thyroid gland include thyroid-stimulating hormone (TSH), free thyroxine (FT4), and free tri-iodothyronine (FT3).[1]
TSH is released by the pituitary gland in response to the blood levels of active thyroid hormones (free T4 and free T3). If the blood thyroid hormone levels are low, the TSH will be elevated and vice versa because of the feedback control mechanism between the thyroid hormones and TSH.[2]
This relationship between TSH and thyroid hormones may be altered due to pathophysiological conditions such as TSH resistance, subclinical hyper- or hypothyroidism, central hypothyroidism, isolated TSH deficiency, and nonthyroid illnesses and a methodological interference during the processing of the sample in the laboratory.[1],[3]
TSH is routinely measured in the clinical laboratory with automated immunoassays. These immunoassays are considered to be gold standard assays as they have high specificity and sensitivity and a short turnaround time. A normal TSH indicates euthyroidism with an accuracy of >99%, except for central hypothyroidism involving a hypothalamic or pituitary disorder.[4]
There has been a significant improvement in immunoassays of thyroid function over the years. For example, third-generation TSH assays have a functional sensitivity of 0.008 mIU/L compared to 1.0 mIU/L with first-generation assays. However, interferences remain a significant problem in thyroid function testing using immunoassay platforms. Troubleshooting these interferences and releasing accurate reports remain a challenge for the laboratory as the interferences may be unique to an individual (e.g., presence of heterophilic antibodies like human anti-mouse antibodies and rheumatoid factors), inducing false-positive or false-negative results.[5]
Case Report
We report a case of a 2-year-old child who was small for gestational age and had poor weight gain. The laboratory received the blood sample for analysis of TSH, free T3, and free T4. The tests were performed in Centaur XP analyzer using Siemens reagents. TSH assay was performed using TSH3-UL (ultra), a third-generation assay for TSH. The result showed that the TSH was very low while FT3 and FT4 were normal. The child was further investigated for thyroid antibodies (antithyroglobulin and antimicrosomal antibodies) and thyroid receptor-stimulating antibodies. An MRI of the brain was done with plasma adrenocorticotropic hormone and insulin-like growth factor-1 to rule out central hypothyroidism. All the results were within normal limits. Clinical reasons could not be ascertained for very low levels of TSH, and hence, it was decided to repeat the thyroid function tests after 3 months. The repeat analysis also gave a similar result for TSH as before. As the results were not correlating clinically, there arose a strong suspicion of methodological interference. The laboratory decided to perform the TSH assay on two other platforms of immunoassay (Beckman and Abbott). TSH values were found to be normal in both of these platforms [Table 1]. A TSH variant was suspected in the child, and a genetic analysis of the TSH gene was ordered.{Table 1}
Genetic analysis
When a whole-exome sequencing of the TSH molecule for the child was performed, a silent nonfunctional mutation of the beta-subunit of the TSH molecule was discovered. The genetic analysis showed a homozygous missense variant in exon 3 of the TSH-beta gene (chr1: g. 115034033A > G; depth: 116x) that results in the amino acid substitution of glycine for arginine at codon 75 (p. Arg75Gly; ENST00000256592.3). It can be postulated that this amino acid substitution in the beta-chain of the TSH molecule (TSH variant) in the child had altered the epitope on TSH, preventing the binding of monoclonal antibodies used in the Siemens TSH Ultra assay to the molecule leading to failure in recognizing the TSH level in the serum. The parents of the child were counseled and assured that the thyroid function of the child was normal. TSH gene analysis of the parents was also recommended.
Discussion
The beta-subunit of TSH is unique and confers specific biochemical and immunological properties to this hormone. The immunoassays used to detect TSH levels in serum use monoclonal antibodies that are directed against a particular epitope in the beta-subunit. This epitope varies with the antibodies used in different commercially available immunoassays for TSH. Any alterations in the amino acid composition of this epitope recognized by the assay antibodies lead to non-binding of the antibodies and hence a very low value of TSH.
Different types of mutations in the TSH-beta gene affecting the function of the TSH molecule resulting in clinical manifestations have been reported in the literature, but there is only one mutation (p. Arg75Gly; ENST00000256592.3) similar to the one observed in our case, known till now which does not affect the functions of TSH but alters its immunoreactivity, causing variable results between assay platforms.[6],[7]
Conclusion
This case reiterates that clinical correlation of laboratory results and effective communication between the clinician and the laboratory are imperative for a correct diagnosis and therapeutic Interventions. It helps to limit the potential errors, avoids unnecessary workup, and sometimes helps in the detection of some very rare genetic disorders like in our case.
Authors' contribution
Dr. Danalakshmi S: Conception of the idea, drafting the article, and final approval of the version to be publishedDr. Mamta Soni: Critical revision of the article and final approval of the version to be published.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
References
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