Thyrotropin Isoforms: Implications for Thyrotropin Analysis and Clinical Practice

Abstract

Serum thyrotropin (TSH) is considered the single most sensitive and specific measure of thyroid function in the general population owing to its negative logarithmic association with free triiodothyronine and free thyroxine concentrations. It is therefore often the test of choice for screening, diagnosis, and monitoring of primary hypothyroidism. Serum TSH concentrations can be analyzed quantitatively using third-generation immunoassays, whereas its bioactivity can be measured by TSH activity assays in cell culture. Theoretically, if serum TSH concentrations are directly related to TSH activity, the two tests should yield comparable results. However, on occasion, the results are discordant, with serum concentrations being higher than TSH biological activity. This review focuses on the dissociation between the clinical state and serum TSH concentrations and addresses clinically important aspects of TSH analysis. Thyrotropin SynthesisT hyrotropin (TSH) is a heterodimeric 28-kDa-glycoprotein hormone released from the anteromedial pituitary and is a regulator of thyroid function. Its synthesis is controlled by the hypothalamic neuropeptide TSH-releasing hormone (TRH). The two peptide subunits of TSH are noncovalently linked and cotranslationally glycosylated with mannose-rich oligosaccharides (1). Posttranslationally, the two subunits are combined and the attached oligosaccharides are further processed. Synthesis of a mature TSH molecule requires the excision of signal peptides from both TSH a-and b-subunits, followed by trimming of mannose and further addition of fucose, galactose, and sialic acids (2). Thus, mature TSH molecules are asparagine-(N)-linked [Asp(N)-linked] complex carbohydrate structures capped with sulfate and/or sialic acid molecules (3,4) (Fig. 1). TSH oligosaccharide structures vary according to the source of TSH: human pituitaryderived TSH comprises fucosylated biantennary glycans with terminal N-acetylgalactosamine sulfate and low sialic acid content (5-7), while recombinant human TSH (rhTSH), synthesized in Chinese hamster ovary cells, specifically terminates in a2,3-linked sialic acid (8-11) and rhTSH formed in yeast lacks sialic acid residues altogether (12). Glycosylation Patterns and TSH BioactivityAppropriate glycosylation is necessary to maintain normal TSH bioactivity (13). Indeed, both a-and b-subunits of TSH have functionally important domains associated with TSH receptor (TSHR) binding and activation. The a-subunit has two Asp(N)-linked oligosaccharide chains with a typical biantennary structure, while TSH b-subunit has only one chain (Fig. 2). The transcriptional and posttranscriptional mechanisms involved in TSH glycosylation result in folding of these subunits, leading to their heterodimerization. These mechanisms also qualitatively regulate TSH secretion, prevent intracellular degradation, and influence TSH clearance rate from the circulation (14,15). A three-dimensional TSH structure has been proposed in the 1990s based on crystallographic studies and comparisons with other glyco...

Is an Isolated TSH Elevation in Chronic Nonthyroidal Illness “Subclinical Hypothyroidism”?

“…The TSH to free T 4 relationship is frequently altered in the ambulatory elderly due to changes independent of thyroid gland failure (14,21,28,39). Two cohort studies have shown that aging is associated with an increase in TSH without a fall in free T 4 (26,27).…”

“…TSH concentrations are currently measured using third-generation immunoassays, which may reflect increased immunoactive but not bioactive TSH, as in the elderly (28). In the future, new technologies for TSH measurements that quantitate only bioactive TSH may help to accurately identify chronically ill patients with mild thyroid gland failure (28).…”

“…In the future, new technologies for TSH measurements that quantitate only bioactive TSH may help to accurately identify chronically ill patients with mild thyroid gland failure (28). Until then, mild thyroid gland failure cannot be reliably diagnosed in chronically ill patients solely by isolated TSH elevations but require follow-up for persistence and progression to overt hypothyroidism (15) ( Figure 1).…”

“…In patients with chronic nonthyroidal illnesses, the hypothalamic-pituitary-thyroid-peripheral-tissue axis and serum thyroid hormone binding are altered, and in vitro TSH and free T 4 estimate values may not reflect in vivo bioactive concentrations, altering the TSH to free T 4 relationship ( Figure 1) (14,28,34,52). The range of TSH values considered to indicate mild thyroid gland failure in otherwise healthy subjects overlaps completely with those associated with nonthyroidal illnesses (Figure 1) (52, 53).…”

Subclinical hypothyroidism: a ‘laboratory-induced’ condition?

“…From a laboratory point of view, the definition of universally applicable TSH-thresholds seems questionable as well, considering the inter-platform variation between TSH assays. As each manufacturer produces distinct TSH-recognizing antibodies, variation in TSH-epitope recognition and binding-affinity is to be expected, which is also influenced by post-translational modifications of TSH (17). Apart from this between-method variation, the TSH and fT 4 reference intervals applied by an individual laboratory are of crucial influence in the correct clinical classification of thyroid conditions.…”