Secretion of alpha subunits of luteinizing hormone (LH) by the anterior pituitary

Medical therapy of gonadotropin-producing and nonfunctioning pituitary adenomas

Clinically nonfunctioning pituitary adenomas are one of the most common types of pituitary tumors. Unless they present with symptoms related to local mass effect, most tumors are detected incidentally when imaging studies are performed for other reasons. Although clinically nonfunctioning, most of these tumors have evidence, in vitro, of gonadotropin hormone or glycoprotein subunit production. The gonadotropins or their monomer submits rarely cause clinically identifiable effects. When these tumors present as macroadenomas, often with associated mass effect and hypopituitarism, primary therapy is neurosurgery. The role for medical therapy will be reviewed here.

Functional uncoupling between intracellular calcium dynamics and secretion in the alphaT3-1 gonadotropic cell line

Gonadotropin releasing hormone (GnRH) stimulates both transcription and secretion of the alpha subunit of the gonadotropins in a Ca2+-dependent fashion. In this study, we examined the role of Ca2+ as the signal coupling agonist occupancy of GnRH receptors to hormone secretion using the gonadotropic cell line alphaT3-1. Treatment of alphaT3-1 cells for 60 min with GnRH (0.1-100 nM), veratridine (50 microM) or high K+ (56 mM) was completely ineffective in stimulating secretion. The lack of effect occurred in spite of a robust, specific, and dose-dependent biphasic [Ca2+]i response consisting of a rapid peak sensitive to thapsigargin (200 nM) followed by a smaller plateau sensitive to the extracellular application of EGTA (5 mM). On the other hand, treatment of alphaT3-1 cells with the Ca2+ ionophore ionomycin resulted in a significant dose-dependent stimulation of secretion and [Ca2+]i responses comparable to those elicited by GnRH. Binding assays revealed the presence of Ins(1,4,5)P3 receptors (Kd = 3.2 nM, Bmax = 50.5 fmol/mg protein) but not ryanodine receptors in alphaT3-1 cell membranes. Together, these results show a functional uncoupling between the [Ca2+]i response and secretion in this cell line, suggesting that the increase in [Ca2+]i triggered by GnRH and depolarization may be necessary but not sufficient to stimulate exocytosis.

Medical therapy for gonadotroph and thyrotroph tumors

Gonadotroph adenomas, a common type of pituitary tumor, are not associated with syndromes of hormonal hypersecretion and thus present as pituitary macroadenomas with mass effects, or as incidentally discovered pituitary masses. When indicated, primary therapy is neurosurgery, but there may be a limited role for medical therapy in patients with residual disease. Thyrotroph adenomas are rare neoplasms that present with hyperthyroidism and local mass effects. Medical therapy may be effective in controlling tumor growth and in achieving euthyroidism, when surgery or radiation, or both, do not control the tumor.

Gonadotropin-releasing hormone receptor mRNA expression by human pituitary tumors in vitro

An important question in the pathogenesis and regulation of human gonadotroph adenomas is whether heterogeneous gonadotropin responses to gonadotropin-releasing hormone (GnRH) are due to dysregulation of GnRH receptor biosynthesis and/or cell-signaling pathways. We investigated gonadotropin responsiveness to pulsatile GnRH in 13 gonadotroph adenomas. All tumors had evidence of follicle-stimulating hormone (FSH) beta and alpha subunit biosynthesis using reverse transcriptase/polymerase chain reaction (RTPCR) techniques. Four tumors significantly increased gonadotropin and/or free subunit secretion during pulsatile 10(-8) M GnRH administration. The GnRH antagonist Antide (10(-6) to 10(-8) M) blocked secretory increases in all GnRH-responsive tumors. Gonadotropin and/or free subunit secretion increased after 60 mM KCl, confirming that GnRH nonresponsiveness was not due to intracellular gonadotropin depletion. We hypothesized that GnRH nonresponsiveness in these tumors may be due to GnRH receptor (GnRH-Rc) biosynthetic defects. RTPCR analyses detected GnRH-Rc transcripts only in responsive tumors and normal human pituitary. This is the first demonstration of a cell-surface receptor biosynthetic defect in human pituitary tumors. We conclude (a) one third of gonadotroph tumors respond to pulsatile GnRH in vitro, (b) GnRH-Rc mRNA is detected in human gonadotroph adenomas and predicts GnRH responsiveness, and (c) GnRH-Rc biosynthetic defects may underlie GnRH nonresponsiveness in gonadotroph tumors.

Gonadotropin evaluation in the diagnosis of polycystic ovary syndrome using either a monoclonal or a polyclonal antibody radioimmunoassay

The LH/FSH ratio values between gonadotropins dosed with a monoclonal antibody assay (IRMA) in the micropolycystic ovary syndrome (PCOS), are discussed and compared to those obtained with the classic assays using polyclonal antibodies. Because of the higher selectivity of this IRMA assay it is noteworthy that the cut-off value between normal and PCOS patients is now equal to or above one. The evaluation of the LH/FSH ratio between the peak values of the two gonadotropins after a GnRH 100 micrograms iv bolus, may be useful in the diagnosis of PCOS in those patients who present an LH/FSH less than 1 in basal conditions even in the presence of clinical and ecographic aspects of PCOS.

[Free alpha-subunit glycoprotein hormones: physiological and pathological data]

The glycoprotein hormone alpha-subunit is secreted as a free molecule as well as a molecule combined to a glycoprotein hormone beta-subunit. In human subjects, plasma levels of the free alpha-subunit were measured by means of a specific radioimmunoassay. Plasma concentrations were high during the neonatal period, then decreased to a nadir at the age of 6 years. A significant pubertal increase occurred in both sexes, more pronounced in girls. In female subjects mean levels (+/- SEM) were 0.21 +/- 0.05 before puberty and 0.51 +/- 0.03 ng 1 degrees IRP-hCG alpha/ml in follicular phase. During menstrual cycle, a typical preovulatory surge was seen simultaneous with the LH surge. During aging, plasma levels increased slowly in males, abruptly in menopausal females. The pituitary reserve as assessed by LH-RH stimulation test (100 micrograms i.v./m2) exhibited a significant pubertal maturation in boys and girls. Chronic administration of LH-RH agonist induced a marked increase of alpha-subunit levels, whereas LH levels were deeply suppressed. LH-RH injections in children treated for precocious puberty with a LH-RH agonist induced a significant release of alpha-subunit despite an almost complete abolition of LH release. In conclusion, from a quantitative point of view, the glyco-protein hormone alpha-subunit is a major secretory product of the pituitary. It seems that there is a specific regulation of its secretion, resembling but not identical to LH secretion regulation. Whether or not it plays a biological role remains uncertain.

Evaluation of the human gonadotroph free alpha-subunit secretory pools by administration of gonadotropin hormone-releasing hormone into normal subjects at different phases of the ovarian cycle

The specificity of a monoclonal antibody RIA for the measurement of free alpha-subunit in plasma is presently documented. This RIA was used to explore the pituitary gonadotroph free alpha-subunit reserve in normal ovulating women stimulated with gonadotropin hormone-releasing hormone (GnRH). RIA specificity was established by means of competitive inhibition curves with various glycoprotein hormone preparations, and Sephadex G-100 exclusion chromatography of purified hFSH (1-2), hLH (LER 960) and pituitary extract. The 3.8% and 5.5% crossreactivities of hFSH 1-2 and hLH LER 960 were shown by exclusion chromatography to result in part from free alpha-subunit contamination in these purified preparations. Pituitary extract chromatography indicated hFSH and hLH cross-reactivity below 2.5% and 1.5%, respectively. Normal females were stimulated with GnRH throughout the cycle: 3 tests were performed on Day 7, 1 test on Day 13, 16, 17 and 22, respectively, and 2 tests on Day 24. GnRH stimulation consisted of an initial 100 micrograms bolus (time 0) followed at 2 h by a 12.5 micrograms/h constant infusion, and a second 100 micrograms bolus at 5 h. In all subjects, baseline free alpha-subunit values were below 2 ng/ml. Total free alpha-subunit secretion was markedly enhanced in subjects Day 13 and 16, in concert with total hLH and hFSH secretion. In the three subjects Day 7, free alpha-subunit was released only after the second GnRH bolus. In periovulatory subjects, free alpha-subunit secretion became apparent after the initial bolus and with constant GnRH infusion. In the three subjects Day 22 and 24, peak levels of free alpha-subunit were obtained after the second GnRH bolus.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and regulation of the pituitary- and placenta-specific human glycoprotein hormone alpha-subunit gene is restricted to the pituitary in transgenic mice

Expression of the glycoprotein hormone alpha subunit occurs in both the pituitary and placenta in humans. However, this study found that expression of this subunit is restricted to the pituitary in mice. An interspecies analysis of human alpha-subunit gene regulation was undertaken, using the transgenic-mouse approach. In mice transgenic for a genomic clone containing the complete human alpha-subunit gene and several kilobases of 5'- and 3'-flanking sequences, cell-type-specific expression and hormonal regulation of the human alpha-subunit transgene occurred in the mouse pituitary, whereas no expression of the transgene was detectable in the mouse placenta. These findings provide strong evidence that a common trans-acting factor(s) regulates glycoprotein hormone alpha-subunit gene expression in the human and mouse pituitaries; however, this factor(s) or a unique factor(s), though functional in the human placenta, is either nonfunctional or absent in the mouse placenta.

Nature of alpha subunit secretion in men: circadian rhythms, pulsatile release and secretory profiles

Alpha subunit complements LH as a marker of the activity of the hypothalamic GnRH pulse generator. To characterize episodes of alpha subunit release and to determine if a circadian pattern of alpha subunit secretion is present in man, spontaneous alpha subunit pulsatility was analyzed in six healthy young men by blood sampling every 5 min for 24 h. The resulting alpha subunit concentration time series were analyzed by two statistically based independent peak detection methods, and subjected to Fourier transformation to assess underlying circadian rhythms. Cross-correlation analyses and multiple parameter deconvolution were used to estimate the concordance of spontaneous and exogenous GnRH-stimulated LH and alpha subunit secretion. These analyses revealed that two independent discrete peak detection algorithms yielded similar estimates of spontaneous alpha subunit pulse frequency, namely, 21 +/- 1.1 (Cluster) and 21 +/- 1.5 (Detect) alpha subunit peaks/24 h. Sampling intensity markedly influenced the estimate of endogenous alpha subunit pulse frequency, inasmuch as estimates from 5-min sampling were significantly greater than those of 10-min or 20-min sampling. Fourier transformation unmasked a significant circadian alpha subunit rhythm in all six men, with maximal concentrations at 0836 h and an average amplitude of 28% of the 24-hr mean hormone concentration. Cross-correlation analysis of spontaneous glycoprotein release revealed that serum LH and alpha subunit concentrations were highly cross-correlated when considered simultaneously, but not at various lags. Finally, deconvolution analysis of exogenous GnRH-stimulated glycoprotein release disclosed distinct half-times of alpha subunit and LH clearance with virtually simultaneous underlying secretory bursts. These data indicate that human alpha subunit is secreted in both a circadian and a discrete pulsatile fashion at a pulse frequency that is significantly underestimated at conventional sampling rates. The approximately hourly alpha subunit interpulse interval (68 +/- 4.6 min) is similar to that reported earlier for LH in peripheral blood and for testosterone in gonadal vein blood in healthy men. Moreover, cross-correlation analysis of endogenous GnRH-driven alpha subunit and deconvolution analysis of exogenous GnRH-stimulated alpha subunit and LH secretion suggest that these glycoproteins are secreted virtually simultaneously, but have significantly different endogenous clearance properties. The remarkably similar in vivo pulse frequencies for alpha subunit, LH, and testosterone in man suggest that the release of these three hormones is coordinately regulated.

Expression and regulation of the pituitary- and placenta-specific human glycoprotein hormone alpha-subunit gene is restricted to the pituitary in transgenic mice

Expression of the glycoprotein hormone alpha subunit occurs in both the pituitary and placenta in humans. However, this study found that expression of this subunit is restricted to the pituitary in mice. An interspecies analysis of human alpha-subunit gene regulation was undertaken, using the transgenic-mouse approach. In mice transgenic for a genomic clone containing the complete human alpha-subunit gene and several kilobases of 5'- and 3'-flanking sequences, cell-type-specific expression and hormonal regulation of the human alpha-subunit transgene occurred in the mouse pituitary, whereas no expression of the transgene was detectable in the mouse placenta. These findings provide strong evidence that a common trans-acting factor(s) regulates glycoprotein hormone alpha-subunit gene expression in the human and mouse pituitaries; however, this factor(s) or a unique factor(s), though functional in the human placenta, is either nonfunctional or absent in the mouse placenta.

Pituitary glycoprotein hormone oligosaccharides: structure, synthesis and function of the asparagine-linked oligosaccharides on lutropin, follitropin and thyrotropin

Luteinizing hormone (LH), follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH) from pituitary and chorionic gonadotropin (CG) from placenta are a family of closely related glycoproteins. Each hormone is a heterodimer, consisting of an alpha- and a beta-subunit. Within an animal species, the alpha-subunits of all four glyco-protein hormones have an identical amino acid sequence, whereas each beta-subunit is distinct and confers hormone-specific features to the heterodimer. LH and FSH are synthesized within the same cell, the gonadotroph of the anterior pituitary, but are predominantly stored in separate secretory granules. We have characterized the asparagine-linked oligosaccharides on bovine, ovine and human LH, FSH and TSH. The various pituitary hormones were found to contain unique sulfated oligosaccharides with the terminal sequence SO4-4GalNAc beta 1----4GlcNAc beta 1----2Man alpha, sialylated oligosaccharides with the terminal sequence SA alpha Gal beta GlcNAc beta Man alpha, or both sulfated and sialylated structures. Despite synthesis of LH and FSH in the same pituitary cell, sulfated oligosaccharides predominate on LH while sialylated oligosaccharides predominate on FSH for all three animal species. We have examined the reactions leading to synthesis of the sulfated oligosaccharides to determine which steps are hormone specific. The sulfotransferase is oligosaccharide specific, requiring only the sequence GalNAc beta 1----4GlcNAc beta 1----2Man alpha. In contrast, the GalNAc-transferase appears to be protein specific, accounting for the preferential addition of GalNAc to LH, TSH, and free (uncombined) alpha-subunits compared with FSH and other pituitary glycoproteins. The predominance of sulfated oligosaccharide structures on LH may account for sorting of LH and FSH into separate secretory granules. Differences in sulfation and sialylation of LH, FSH and TSH may also play a role in the regulation of hormone bioactivity.

Characteristics of radioimmunoassays for the alpha- and beta-subunits of human luteinizing hormone

The binding characteristics and specificities of the National Hormone and Pituitary Program (NHPP) kits for the radioimmunoassay of the alpha- and beta-subunits of human luteinizing hormone (hLH-alpha and hLH-beta) were studied, as well as the specificities of the anti-hLH and anti-human follicle stimulating hormone (anti-hFSH) antisera distributed by the same organization. The affinity constants of the anti-hLH-alpha and anti-hLH-beta antisera were calculated at 157 +/- 8.4 nM-1 and 109 +/- 7.4 nM-1, respectively. Both antisera were highly specific with regard to the other subunit. However, in the homologous hLH-alpha RIA, native hLH cross-reacted at 21.9%, hFSH at 17.5% and hTSH at 7.9%. The alpha-subunit of the human chorionic gonadotropin, hCG-alpha, was equipotent with the hLH-alpha standard in this assay. In the homologous hLH-beta RIA, hLH showed a cross-reactivity of 14.7% while the cross-reactivities of hCG-beta, hFSH and hTSH were 3.5%, 1.2% and 0.6%, respectively. The anti-hFSH antiserum was highly specific, while the anti-hLH antiserum showed non parallel competition curves. With this knowledge of the specificity of each antiserum, corrections can be properly made for the assays of hLH, hLH-alpha and hLH-beta while the hFSH RIA can be used without correction for the presence of the three other components.

Free alpha-subunit response to gonadotropin-releasing hormone in women with polycystic ovaries

The response of glycoprotein hormone free alpha-subunit to gonadotropin-releasing hormone (GnRH) was evaluated in 12 women with polycystic ovaries (PCOs). Six of these women were premedicated for 3 days with micronized 17 beta-estradiol before receiving a 100-micrograms bolus of GnRH. In nonmedicated PCO patients, GnRH did not significantly alter basal free alpha-subunit levels. In four of the six PCO patients receiving estrogen premedication, a significant increase in free alpha-subunit was observed; these four patients had low progesterone levels at the time of the GnRH test. Among the six premedicated patients, two had elevated (greater than 4 ng/ml) progesterone levels, and the GnRH tests showed no significant effect on the levels of free alpha-subunit. The study revealed a dissociation between the free alpha-subunit responses to GnRH and the responses of luteinizing hormone; a closer relationship was observed between free alpha-subunit and follicle-stimulating hormone responses. It was concluded that the lack of a free alpha-subunit response to GnRH in PCO patients is not due to a primary inability of the pituitary gonadotroph to produce free alpha-subunit but is a consequence of an altered estrogenic milieu, and a free alpha-subunit response to GnRH may reflect the replenishment of both follicle-stimulating hormone and luteinizing hormone in the gonadotrope.

Differential processing of Asn-linked oligosaccharides on pituitary glycoprotein hormones: implications for biologic function

Luteinizing hormone, follicle-stimulating hormone, and thyroid-stimulating hormone from pituitary and chorionic gonadotropin from placenta are a family of glycoproteins, each consisting of an alpha and beta subunit. Within an animal species, the alpha subunit of all four hormones contains the identical amino acid sequence, while each beta subunit is distinct and confers biologic specificity to the hormone dimer. Despite sharing common alpha subunits, these hormones bear Asn-linked oligosaccharides which differ in structure. Whereas chorionic gonadotropin contains exclusively neutral and sialylated oligosaccharides, the pituitary hormones bear neutral, sialylated, sulfated, and sialylated/sulfated structures. The sulfated oligosaccharides are unique in structure and are more prevalent on certain pituitary hormones, indicating that the synthesis of these unusual oligosaccharides is tightly regulated. The differences in oligosaccharide structures in conjunction with the highly specific endocrine responses elicited by these hormones, suggest an important functional role for the oligosaccharides, such as metabolic clearance, control of hormone response, modulation of hormone potency, and/or intracellular sorting of hormones into separate secretory granules.

Induction of pituitary lactotrope differentiation by luteinizing hormone alpha subunit

Addition of gonadotropin releasing hormone to cultures of fetal rat pituitary induced differentiation of lactotropes as revealed by immunocytochemistry. Antiserum to luteinizing hormone (LH) (recognizing native LH), but not antiserum to LH-beta (recognizing both native LH and its beta subunit), inhibited this induction. Further addition of highly purified LH-alpha subunit in culture medium also induced lactotrope differentiation. Thus, the alpha subunit may have a specific biological activity of its own with probable practical use in clinical investigations.

Quantitation of LH subunits released by rat anterior pituitary cells in primary culture

Immunoreactive rLH, rLH alpha, rLH beta and rFSH in the cell extracts and medium of rat anterior pituitary cell cultures were measured after gel filtration on Sephadex G-100 Superfine. Cell extracts contained uncombined rLH alpha and rLH beta present as approximately 40% and 4%, respectively, of native rLH on a molar basis. Unstimulated cultures appeared to release a large excess of rLH alpha (approximately 2560% of rLH on a molar basis) as well as a minimum of uncombined rLH beta. Stimulation of cultures with physiological (LHRH) or non-physiological agents (A23187, elevated K+) increased the absolute but not the relative quantities of uncombined subunits released relative to those present intracellularly. Thus, rat anterior pituitary cell cultures appear to produce and release an excess of free alpha subunit as well as a minimum of uncombined rLH beta. However, one cannot be certain whether the uncombined rLH beta represents a pool of 'free' rLH beta or simply results from the dissociation of rLH.

A role for luteinizing hormone and follicle-stimulating hormone in the etiology of simple goiters in the pediatric population

In light of the progressive, pubertal increases in serum gonadotropin (FSH, LH) levels coinciding with the peak incidence of simple goiters; the common alpha subunits associated with the gonadotropins, TSH and hCG; the ability of LH and hCG to bind to thyroid TSH receptors and affect thyroid function; and, the probable interactions between thyroid hormones and gonadotropins, a role for gonadotropins in the etiology of simple goiters merits consideration.

Control of gonadotrophin secretion by steroid hormones in male castrates: site of oestradiol action

Four normal healthy castrated male transsexuals who were not on hormone therapy for at least 4 months volunteered for this study. Each subject received 3 infusions at weekly intervals--the first consisting of LHRH alone, the second of LHRH and oestradiol and the third of oestradiol alone. Venous blood samples were collected at 20-minute intervals 1 hour before the start and during the whole period of infusion. Plasma concentrations of FSH, LH and oestradiol were measured by radio-immunoassay. On an average, 100 micrograms of LHRH enhanced the area under the LH secretion curve by 177% and under the FSH curve by 68%. Oestradiol had a differential effect on the endogenous secretion of LH and FSH, suppressing them by 53% and 18%, respectively. Similarly, the LHRH-stimulated release of LH was significantly inhibited to a greater extent (42%) than that of FSH (23%, P less than 0.05; paired t test). Such an observation and in the light of our acceptance of the single releasing hormone theory, suggests that the differential actions of oestradiol on the secretions of FSH and LH are due to mechanisms residing in the pituitary.

Serum glycoprotein hormone alpha subunit levels in patients with cancer

The serum glycoprotein hormone alpha subunit concentration was measured in 957 nonpregnant patients with benign disorders and 683 patients with unselected malignancies. Postmenopausal women had significantly higher alpha levels than premenopausal women or men. When the patients were subdivided according to age, sex or disease sites, significant population differences were found for women less than 50 years of age and patients with cancers of presumed neural crest origin. However, individual serum alpha levels in patients with benign disorders or malignancies demonstrated considerable overlap. No population differences in serum alpha concentrations were demonstrated between patients grouped according to stage of disease, suggesting that serum alpha were not directly related to tumor burden. Similarly, there was no statistical association between clinical improvement or deterioration and change in the serum alpha subunit concentration. These results indicate that measurement of the glycoprotein hormone alpha subunit concentration in the serum is not useful for screening patients for cancer or for monitoring the clinical course of patients with the vast majority of cancers.

Immunoreactive alpha and beta subunits of follicle stimulating and luteinizing hormones in peripheral blood throughout the menstrual cycle and following stimulation with synthetic gonadotropin releasing hormone (GnRH)

The concentration of immunoreactive alpha and beta subunits of human LH (hLH) and human FSH (hFSH) were measured in unfractionated serum throughout normal menstrual cycles, and when GnRH was administered at about midcycle. The RIA subunit systems employed were sufficiently sensitive and specific to measure basal levels of free alpha-hLH, beta-hLH, and alpha-hFSH. The beta-hFSH system lacked sufficient sensitivity to measure beta-hFSH in unfractionated serum. In normal cycles, alpha-hLH, beta-hLH and alpha-hFSH were shown to circulate in the free form with highest concentrations occurring at the time of midcycle peak of LH and FSH. The concentration of subunits during the follicular phase were not significantly different from those observed during the luteal phase of the cycle. In response to GnRH administration during the periovulatory phase of the cycle, the increases in circulating free alpha-hLH, beta-hLH, and of alpha-hFSH subunits were directly related to the increases in LH and FSH. Other investigators have shown that increases in circulating levels of free subunits are due to secretion from the pituitary gland rather than from peripheral dissociation of intact hormone. Therefore, it can be assumed that the increase in immunoreactive alpha-hLH, beta-hLH, and of alpha-hFSH subunits observed in this study at midcycle represent release of free subunits in response to endogenous or exogenous GnRH administration.

Ectopic secretion of chorionic gonadotropin by a lung carcinoma. Pituitary gonadotropin and subunit secretion and prolonged chemotherapeutic remission

The ability of tumor markers to improve cancer therapy is not established. We studied a man with a human chorionic gonadotropin (HCG)-secreting large cell carcinoma of the lung and gynecomastia. Preoperatively, levels of HCG (109 ng/ml), its alpha and beta subunits (3.2 and 21 ng/ml, respectively) and plasma estradiol were elevated. Despite apparently complete tumor resection and total resolution of gynecomastia, HCG titers remained elevated (3.3 ng/ml), heralding tumor recurrence three weeks later. Because the pathophysiologic consequences of the ectopic secretion of HCG on pituitary function are not established, we administered 100 microgram of gonadotropin-releasing hormone (LHRH) and observed a markedly delayed increase in pituitary gonadotropins. Early chemotherapy, guided by persistence of HCG, reduced HCG to undetectable levels, restored to normal the response to LHRH and resulted in a distinctly unusual 30-month complete remission. Use of HCG as a tumor marker levels is more sensitive than the symptom of gynecomastia and may permit detection of small, potentially curable tumor foci.

Metabolic clearance and secretion rates of subunits of human thyrotropin

Metabolic clearance rates (MCR) of the alpha and beta subunits of human thyrotropin (hTSH-alpha and hTSH-beta) were determined by a constant infusion to equilibrium method. In 15 normal individuals (six men, six premenopausal women, and three post-menopausal women), the mean MCR of hTSH-alpha (68 ml/min per m2) was significantly faster than that of hTSH-beta (48 ml/min per m2) was significantly faster than that of hTSH-beta (48 ml/min per m2); both were two to three times more rapid than the previously determined MCR of hTSH. In patients with primary hypothyroidism, MCR were significantly slower with a mean value of 55 ml/min per m2 for hTSH-alpha and 37 ml/min per m2 for hTSH-beta. However, MCR of subunits were not significantly faster than normal in hyperthyroid patients. Serum concentrations of alpha subunits and hTSH-beta were measured by radioimmunoassay, and secretion rates of alpha and hTSH-beta from the pituitary were calculated using hTSH-alpha and hTSH-beta MCR, respectively. In the normal individuals, alpha secretion rates averaged 91 mug/day per m2, greater than those previously determined for hTSH and human follicle-stimulating hormone. Alpha secretion rates were significantly elevated in the normal postmenopausal women (211 mug/day per m2) and in the premenopausal hypothyroid women (202 mug/day per m2); they were also elevated in the postmenopausal hypothyroid women (277 mug/day per m2). Alpha secretion rates were significantly decreased in the premenopausal hyperthyroid women (66 mug/day per m2). Usually, the secretion rates of hTSH-beta could not be calculated in normal individuals, and the rates in hyperthyroid patients could never be calculated because serum hTSH-beta was not detected. Six normals had detectable hTSH-beta secretion rates (17 mug/day per m2); hTSH-beta secretion rates were significantly increased in patients with primary hypothyroidism (28 mug/day per m2). Although we had previously demonstrated a 50-fold increase in hTSH secretion rates in primary hypothyroidism, there was only a 2-fold increase in alpha and hTSH-beta secretion rates. Thus, increased subunit synthesis appears to be utilized predominantly for production of complete hTSH.