BMJ | 2021
Different thyroid assays may greatly affect diagnosis and management of hypothyroidism
Abstract
Siskind and colleagues do not mention the impact of thyroid assays and reference ranges on the diagnosis and management of hypothyroidism.1 It is generally believed that assay differences are accounted for by assay specific reference intervals. We found, however, that the diagnosis and management of subclinical hypothyroidism (SCH) is strikingly different depending on thyroid stimulating hormone (TSH) and free thyroxine (fT4) assays provided by Abbott Laboratories and Roche Diagnostics, employed by 75% of clinical laboratories in the UK.2 We identified 53 consecutive patients with Roche defined SCH, of whom 40 (75.5%) had normal thyroid function and 13 (24.5%) had SCH when analysed with Abbott assays. We also identified 40 consecutive patients with Abbott defined SCH of whom 28 (70%) had SCH and 12 (30%) had results indicative of levothyroxine replacement as per NICE guidance3 when analysed with Roche assays. Only 44% of patients had concordant results with both methods.2 The results were interpreted using manufacture provided, assay specific reference intervals which most laboratories use. Compared with Abbott TSH results, the Roche TSH results were 40% higher but the upper reference limit was 18% lower; therefore, it reported a greater number of high TSH results. Compared with the Abbott fT4 assay, Roche fT4 results were 16% higher but the lower reference limit was 25% higher; therefore, it reported a greater number of low fT4 results. It is, however, uncertain whether Roche assays lead to incorrect diagnosis and treatment of SCH or Abbott assays lead to missed diagnosis and undertreatment of SCH. Monitoring of levothyroxine replacement in primary hypothyroidismusingAbbott andRocheTSHassaysmayalso result indifferent clinicalmanagementdecisions in 14% of patients.4 Clinicians should be aware that assay differences and variations in reference ranges will impact the diagnosis and management of hypothyroidism.