Current utility of first-line FT4 and TSH in screening for central hypothyroidism

Evans, Carol, Jacob, Jiya, Rodham, Annabel, Gill, Manjot, Parry, Laura, Dodd, Alan, El Farhan, Nadia, Shore, Angharad, Lansdown, Andrew, Rees, Aled ORCID: https://orcid.org/0000-0002-1165-9092 and Okosieme, Onyebuchi 2024. Current utility of first-line FT4 and TSH in screening for central hypothyroidism. Clinical Endocrinology 10.1111/cen.15068

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Abstract

Background Thyroid testing strategies vary across laboratories. First-line combined thyroid stimulating hormone (TSH) and freeT4 (FT4) have historically been preferred by many laboratories as this detects individuals with undiagnosed central hypothyroidism who can be missed with a first-line TSH-only strategy. However, an up-to-date evaluation of the utility of this approach is lacking. Objectives We investigated the clinical utility of first-line TSH and FT4 in the detection of central hypothyroidism in current day practice. Design, Patients, and Measurements The All-Wales laboratory information system was queried to identify thyroid function tests in patients aged ≥16 years with decreased FT4 and inappropriate TSH (low-FT4). The 1-year incidence of low-FT4 was determined using mid-year population data. Clinical information of patients with low-FT4 was reviewed to determine causes of low-FT4 and the incidence of central hypothyroidism. Results The incidence of low-FT4 varied according to FT4 assay method (range: 98–301 cases/100,000 population/year). Fifteen new cases of central hypothyroidism were detected in two health boards, equivalent to 2 cases/100,000 population/year. Positive predictive value of low-FT4 for central hypothyroidism was 2%–4%. In a cross-section of primary care patients, low-FT4 was detected in 0.5% of all thyroid tests with assay-related differences in detection rates. Conclusions Although low-FT4 is a common laboratory finding, the incidence of central hypothyroidism remains rare. With the currently increased rates of thyroid testing and increased use of medications that decrease FT4, low-FT4 has a much lower predictive value for central hypothyroidism than previously reported. Thyroid screening strategies will need to balance the yield from first line TSH and FT4 testing with the cost of investigating individuals with non-pathological laboratory abnormalities.

Item Type:	Article
Date Type:	Published Online
Status:	In Press
Schools:	Medicine Neuroscience and Mental Health Research Institute (NMHRI) MRC Centre for Neuropsychiatric Genetics and Genomics (CNGG)
Publisher:	Society for Endocrinology
ISSN:	1365-2265
Date of First Compliant Deposit:	1 May 2024
Date of Acceptance:	19 April 2024
Last Modified:	03 May 2024 10:00
URI:	https://orca.cardiff.ac.uk/id/eprint/168557

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