Secretion of Alpha and Beta Subunits of TSH by the Anterior Pituitary

Abstract

We have attempted to determine whether blood concentrations of the alpha chains of TSH arise by direct pituitary secretion of alpha chain or by degradation of the intact hormone after secretion. Highly purified human TSH was administered by infusion to euthyroid volunteers; blood TSH concentrations were raised to 36 muu/ml. After TSH infusion, circulating blood alpha chain concentrations did not change. The same volunteers received an infusion of TRH which stimulated pituitary secretion of TSH, raising blood concentrations to 15 muu/ml in one series of subjects and to 23 muu/ml in a second series. After TRH, blood concentrations of alpha chain increased from undetectable to over 400 pg/ml. We conclude that alpha subunits of TSH are directly secreted by the pituitary gland and are not formed by degradation of intact TSH in the peripheral circulation.

Dynamics and Regulation of TSH Secretion by Superfused Anterior Pituitary Cells

“…It is possible, how ever, that the biphasic nature of the response as measured by radioimmu noassay is not totally reflective of the biologically active TSH release. Sev eral laboratories [5,8,16] have reported release of a-and (3-TSH subunits after TRH administration. The release of TSH subunits in response to TRH may contribute to the biphasic nature of the response as measured by radioimmunoassay.…”

Characterization and nucleotide sequence of the gene for the common α subunit of the bovine pituitary glycoprotein hormones

“…Characteristically, intracellular levels of pituitary free a subunit exceed the intracellular levels of mature glycoproteins (5)(6)(7)(8)(9). This imbalance has led to the hypothesis that assembly of the hormones is limited by the appearance of each B subunit and further, that synthesis of the a and B subunits are independently regulated (10)(11)(12).…”

Metabolic clearance and secretion rates of subunits of human thyrotropin.

“…We have previously reported that both the alpha subtunit and hTSH-p8 are present in the serum of patients with primary hypothyroidism and are secreted from the pituitary I Abbreviations used in this paper: alpha or a, immunologically common alpha subunit of the glycoprotein hormones; fT4, free thyroxine; hCG, human chorionic gonadotropin; hCG-a, alpha subunit of hCG; hFSH, human follicle-stimulating hormone; hLH, human luteinizing hormone; hTSH, after the administration of thyrotropin-releasing hormone (TRH) (2,5). In addition, normal postmenopausal women have higher serum alpha concentrations than premenopausal women or men (2, 6); furthermore, serum alpha concentrations in normal individuals demonstrate greater increases after luteinizing hormone-releasing hormone (LHRH) than after TRH (7)(8)(9)(10).…”