Thyrotropin-releasing hormone time-dependently influences thyrotropin microheterogeneity--an in vivo study in euthyroidism

Abstract

Thyrotropin (TSH) is secreted not as one distinct hormone, but rather as a group of isohormones which differ in their oligosaccharide composition. Although the mechanisms regulating TSH glycosylation are not fully understood, there is strong evidence that TRH plays an important role. The aim of our study was to determine the dynamic influence of TRH on TSH microheterogeneity.Sera were obtained from euthyroid volunteers (n=20) before and 30, 60, 120, 180 and 240 min after intravenous, nasal and oral administration of TRH in three independent runs (randomized order, at a time-interval of 3 weeks between each run). TSH was immuno-concentrated and analysed by isoelectric focusing (IEF) and lentil lectin affinity chromatography. TSH immunoreactivity was measured by an automated second-generation TSH immunoassay. Overall, serum TSH concentrations reached maximal values 30 min after intravenous, 60 min after nasal and 180 min after oral TRH stimulation. IEF analysis revealed 63·3 3·3% of pituitary standard TSH (IRP 80/558) in the neutral pH range (8>pH>6). In contrast, 30 min after TRH stimulation 80·8 3·7% (P<0·001) and 60 min after TRH stimulation 44·9 2·2% (P<0·001) of the TSH of euthyroid probands were found in this pH range, whereas 180 min after TRH stimulation 58·4 2·3% (P<0·001) were detected in the acidic pH range (pH<6). This shift of TSH composition in euthyroidism after TRH stimulation was confirmed by lentil lectin analysis of TSH: core-fucose content of euthyroid TSH was 73·4 3·8% 30 min and 22·9 3·2% 120 min after TRH stimulation in contrast to basal (53·3 1·8%; P<0·001) and pituitary standard (IRP 80/558) TSH (63·0 0·9%; P<0·001).In conclusion, in euthyroidism, TRH stimulation timedependently changes the distribution pattern of the TSH isoforms from an alkaline and neutral to a more acidic one. This corresponds to the secretion of isohormones with altered bioactivity which could influence the fine-tuning of thyroid function.

Thyroid-Stimulating Hormone and Thyroid-Stimulating Hormone Receptor Structure-Function Relationships

“…Initially, physicochemical and enzymatic studies have identified amino acids as well as carbohydrate portions on both subunits which contribute to dimerization of two subunits, receptor binding, and signal transduction. These approaches, which are summarized in excellent reviews (5,14,39,140,171,187), have been instrumental in gaining an initial understanding of glycoprotein hormone structure-function relationships and continue to provide valuable information to the present time. Other valuable strategies rely primarily on epitope mapping or the use of synthetic peptides (39,88,94,129).…”

“…Very recently, sera obtained from euthyroid volunteers before and 30, 60, 120, 180, and 240 min after intravenous, nasal, and oral administration of TSH were found to contain increased acidic forms as a function of time (171). Lectin analysis confirmed a concomitant increase in core-fucosylated glycans.…”

Standardisation des immunodosages de la TSH : production de nouveaux calibrateurs et harmonisation des tests

“…2D). Plus récemment, des échantillons sériques de sujets euthyroïdiens obtenus avant et après administration de TRH se sont révélés contenir une quantité croissante de formes acides avec le temps [51]. Par ailleurs, des études faisant appel aux chromatographies d'affinité sur lectine insolubilisée ont permis de montrer que la TSH contiendrait moins de résidus de fucose interne chez des patients atteints d'hypothyroïdie infraclinique [50], ainsi qu'après administration de TRH [51], comparée à des sujets euthyroï-diens.…”