Prevalence and Clinical Characteristics of Macro-TSH in Patients with Subclinical Hypothyroidism

Macro-thyrotropin (TSH) is a high molecular weight complex of TSH associated with apparently elevated levels of serum TSH, which can lead to an incorrect diagnosis of subclinical hypothyroidism. Given that levothyroxine (LT4) replacement is unnecessary in such cases, the accurate identification of macro-TSH based on clinical and laboratory findings is of particular importance. We aimed to determine the prevalence and clinical features of macro-TSH in patients with subclinical hypothyroidism evaluated after the introduction of TSH harmonization.

Between August and November 2023, we included 1599 consecutive patients with subclinical hypothyroidism (TSH > 4.23 mIU/L with normal fT4 levels) who were either untreated or receiving LT4 replacement monotherapy. Polyethylene glycol (PEG) precipitation was performed in all cases. Patients with TSH recovery of <20% underwent further evaluation, including gel filtration chromatography under neutral and acidic conditions, protein G and Jacalin precipitation assays, and a comprehensive battery of interference tests.

Four patients (0.25%) showed high molecular-weight TSH peaks (>150 kDa) under neutral conditions that disappeared under acidic conditions, with a shift to the native molecular weight (approximately 28 kDa), indicating the initial presence of macro-TSH. The TSH complex was detected in peaks corresponding to IgG, IgA, and IgM. These findings were verified using Protein G and Jacalin precipitation assays, with no evidence of assay interference. Reevaluation of TSH recovery following PEG precipitation showed substantial variation in two patients who had abundant monomeric TSH compared with the high-molecular complex. All four patients with macro-TSH had previously received LT4 therapy, which resulted in a modest reduction in TSH levels, with three of these patients testing positive for TgAb and/or TPOAb.

Macro-TSH complexes comprise several immunoglobulin isotypes. In cases with low antibody-TSH binding affinity and positive antithyroid autoantibodies, TSH levels may fluctuate during the clinical course, thereby complicating diagnosis and management.