Elevated histaminase activity in medullary carcinoma of the thyroid gland

RET gene mutation analysis and long-term clinical outcomes of medullary thyroid cancer patients

OBJECTIVE

Medullary thyroid carcinoma is a rare, potentially aggressive tumour, with relatively worse prognosis than well-differentiated thyroid cancer. We evaluated the long-term outcomes and prognosis of medullary thyroid carcinoma patients at a single institution in India and compared outcomes based on results of RET protooncogene mutation analysis.

METHODS

Data were retrieved through a prospectively maintained thyroid cancer database from 1998 to June 2019, and medullary thyroid carcinoma patients were recruited. RET gene mutation status (exon 10-16) was assessed. Patient with a minimum follow-up of 12 months was eligible to be part of the long-term outcome analysis.

RESULTS

Out of 149 peripheral blood samples, 42 were positive for RET gene mutation (prevalence of 28.1%). The median follow-up duration was 48 months, ranging from 12 to 240 months. Long-term clinical outcomes of 113 patients were assessed. Two deaths were noted in this series. Both 5- and 10-year survival was cent per cent. Overall survival was 98.2% (97.3% in RET positive and 98.7% in RET negative group). Progression-free survival was 55.4% in total (60% in RET positive and 53.3% in RET negative group). No statistically significant difference was found between RET positive and RET negative groups concerning overall survival (P = 0.6011) and progression-free survival (P = 0.5140). Univariate analysis revealed high calcitonin (>10 pg/mL), stage IV disease, and presence of lymph nodal metastasis to be significant predictors of disease recurrence, however, multivariate analysis demonstrated the presence of lymph node metastases as the only significant predictor of recurrence (P = 0.0005).

CONCLUSIONS

Medullary thyroid carcinoma patients had relatively favourable long-term outcomes. Long-term survival was similar irrespective of RET mutation status. Presence of lymph node metastases appeared to be the strongest predictor of overall and progression-free survival, followed by Calcitonin level and stage of the disease.

Revised American Thyroid Association guidelines for the management of medullary thyroid carcinoma

INTRODUCTION

The American Thyroid Association appointed a Task Force of experts to revise the original Medullary Thyroid Carcinoma: Management Guidelines of the American Thyroid Association.

METHODS

The Task Force identified relevant articles using a systematic PubMed search, supplemented with additional published materials, and then created evidence-based recommendations, which were set in categories using criteria adapted from the United States Preventive Services Task Force Agency for Healthcare Research and Quality. The original guidelines provided abundant source material and an excellent organizational structure that served as the basis for the current revised document.

RESULTS

The revised guidelines are focused primarily on the diagnosis and treatment of patients with sporadic medullary thyroid carcinoma (MTC) and hereditary MTC.

CONCLUSIONS

The Task Force developed 67 evidence-based recommendations to assist clinicians in the care of patients with MTC. The Task Force considers the recommendations to represent current, rational, and optimal medical practice.

No effect of cimetidine on calcitonin secretion from medullary thyroid carcinoma

The effect of cimetidine on basal and pentagastrin-stimulated serum immunoreactive calcitonin (S-iCT) concentrations was studied in six patients with medullary carcinoma of the thyroid (MCT). Basal S-iCT was elevated in all patients and showed a marked increase after i.v. injection of pentagastrin, 0.5 micrograms/kg b.wt. Cimetidine, 200 mg i.v., 30 min before administration of pentagastrin had no effect on either basal or pentagastrin-stimulated S-iCT. Thus, the mechanisms of basal and pentagastrin-stimulated calcitonin secretion from MCT do not seem to involve agonism with histamine H2-receptors.

Hypercalcitoninaemia in a patient with urticaria pigmentosa. A possible cause of diarrhoea

Mastocytosis gives rise to clinical symptoms such as flushing, itching and diarrhoea. We report a patient with urticaria pigmentosa without evidence of systemic involvement but with recurrent episodes of diarrhoea. The patient had elevated circulating levels of calcitonin, which might have been a mediator of her diarrhoea. We suggest that serum calcitonin level should be checked in patients with mast cell disease and diarrhoea.

Ectopic secretion of calcitonin

In 7 out of 25 unselected patients (28%) with a malignant disease, the circulating calcitonin level had increased. Pentagastrin i.v. gave no elevation of the calcitonin level comparable to the response in patients with medullary carcinoma of thyroid.

Urine GABA levels in ovarian cancer patients: elevated GABA in malignancy

The association of malignancy with elevated diamine oxidase (DAO), an enzyme producing gamma-aminobutyric acid (GABA) is well documented. In ovarian cancer, increased DAO occurs in the malignant tissues and plasma. Since higher DAO levels cause GABA accumulation, elevated GABA may occur in ovarian cancer and be reflected in urine. We tested this hypothesis and found that half the ovarian cancer patients had a clearly elevated urine GABA, a result that is in agreement with previous reports on DAO and malignancy. The published findings on DAO, GABA and malignancy suggest that elevated GABA is associated with cancer. This proposal could lead to a GABA urine monitor or new directions in cancer treatment or research.

Aminoguanidine does not inhibit aldose reductase activity in galactose-fed rats

Aminoguanidine, nucleophilic hydrazine derivative, has been shown to inhibit diamine oxidase, the formation of advanced glycation endproducts, nitric oxide synthase, and catalase. Prompted by the reports that aminoguanidine also inhibits aldose reductase (AR), we have investigated the effect of aminoguanidine, 1,3-diaminoguanidine, and methylguanidine on AR activity in vitro, and in vivo. In vitro, we have measured the inhibition of AR isolated from bovine lenses; in vivo, we have examined the effect on the galactitol levels in the red blood cells, sciatic nerve, retina, and lens of rats administered the test compounds for 11 days in the drinking water and, for the last 4 days, given access to a 20% galactose diet. Two known, structurally distinct AR inhibitors, tolrestat and compound WAY-121,509, were used as reference. In vitro, at concentrations up to 1.0 mmol/L, none of the tested guanidine derivatives had any effect on AR. As a corollary, in vivo, at doses ranging from 201 to 349 mg/kg/day, none of the guanidine derivatives affected tissular galactitol levels. We conclude that, in short-term galactose-fed rats, at the doses tested, aminoguanidine, 1,3-diaminoguanidine, and methylguanidine do not inhibit AR.

Diabetic late complications: will aldose reductase inhibitors or inhibitors of advanced glycosylation endproduct formation hold promise?

Patients suffering from the severe complications associated with both insulin- (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM): nephropathy, retinopathy, neuropathy, and atherosclerosis are still largely left without a prospect of an efficient treatment. This is the case even if it has been assumed for decades and now finally proved by the results from the Diabetes Control and Complications Trial (DCCT) that hyperglycemia is the single main cause of these complications. Improved glycemic control as a result of intensive insulin treatment has the potential to reduce the incidence and progression of complications, but implementation and monitoring of improved glycemic control in all groups of IDDM and NIDDM patients in different communities will be difficult and expensive. Results from the recently terminated DCCT have shown that even with intensive insulin treatment, there will be a significant burden of complications on the diabetic population. It will, therefore, still be of immense importance for the long-term quality of life for the diabetic patient that additional possibilities are developed for prevention and intervention against diabetic complications. Almost two decades of research, animal model testing, and clinical trials have been conducted on various efficient aldose reductase inhibitors. Now the concept of inhibition of formation of advanced glycosylation endproducts on proteins and lipids resulting from extra- and intracellular hyperglycemia is entering the scene as an alternative or perhaps supplementary approach to reduce the occurrence of diabetic complications. An overview of the results from these two fields of research and associated drug-development programs will be presented along with thoughts on possible future developments.

Diamine-oxidase activity and tissue di- and poly-amine contents of human ovarian, cervical and endometrial carcinoma

In view of the conflicting reports regarding the association of di- and poly-amines along with diamine oxidase (DAO) activity in human carcinomatous growths, the present study was undertaken to establish their interrelation and to justify whether analysis of di- and poly-amine contents along with DAO may have any diagnostic value in the assessment of the carcinomatous state. It was found that diamines like histamine, putrescine and cadaverine, and polyamines like spermine and spermidine, and also DAO, increased unequivocally in human ovarian, cervical and endometrial carcinoma in comparison with their adjoining normal tissues. A reasonable explanation has been put forward to support the observations made.

Medullary thyroid carcinoma

Medullary thyroid carcinoma accounts for 5-10 per cent of thyroid malignancies and occurs sporadically, and in three familial settings with autosomal dominant inheritance. Calcitonin, a 32 amino acid 3.5-kDa protein, is a biochemical marker of the tumour. Serum levels correlate with disease burden. Treatment initiated by screening allows disease removal at a premalignant phase: C cell hyperplasia. Genetic linkage studies locate the multiple endocrine neoplasia type 2A gene near the centromere of chromosome 10. Flanking genetic markers may allow polymorphic probes to examine DNA from a once-only blood sample to identify affected individuals. At what age thyroidectomy should be carried out in these patients is unclear.

Medullary carcinoma of the thyroid: current diagnosis and management

Medullary thyroid carcinoma (MTC) accounts for 5-10% of thyroid malignancies and occurs in either a sporadic or a familial form. The familial form is inherited in an autosomal dominant pattern, and expressed clinically as multiple endocrine neoplasia (MEN), types IIa and IIb, or as familial MTC alone. This neoplasm is derived from the parafollicular or C-cells, and has the ability to secrete a variety of polypeptide hormones including calcitonin, which serves as a tumor marker for the presence of MTC. The development of a calcitonin radioimmunoassay and the screening of patients at risk for the familial forms of MTC allows the diagnosis of the neoplasm in an occult stage when total thyroidectomy results in virtually 100% cure. We will present our experience with the diagnosis, treatment, and postoperative follow-up of our patients with this interesting neoplasm.

Melanin production in black medullary thyroid carcinoma (MTC)

Melanin production by two medullary carcinomas of the thyroid is reported and discussed. In both tumours, melanin and calcitonin could be detected in the same cells.

Malignant disease: nutritional implications of disease and treatment

Protein-energy malnutrition (PEM) is common in cancer patients and may develop into the syndrome known as 'cancer cachexia'. This is characterised by complex disturbances in carbohydrate, lipid, protein, and electrolyte metabolism. The aetiology is equally complex, with host and therapeutic factors contributing to the reduced food intake and effects on host tissues. Anorexia is of prime importance, differing in its cause from one patient to another and often presenting a barrier to successful nutritional support. Further research is necessary to elucidate the interaction of central and peripheral factors that may be involved in the aetiology of anorexia. Because of the interplay of biochemical, physiological, and psychological consequences of cancer, the nutritional support of the patient presents a considerable challenge to the caring professions.

Advances in the diagnosis and treatment of medullary thyroid carcinoma

Medullary thyroid carcinoma accounts for 5 to 10 per cent of all thyroid malignancies and may occur in a familial or a sporadic pattern. This article reviews the authors' experience with 200 patients with medullary thyroid carcinoma and outlines the recent advances made in our understanding of the biochemical properties of these cancer cells and the relationship of different tumor markers to prognosis.

Diamineoxidase activity and tissue histamine content of human skin, breast and rectal carcinoma

In view of the controversy that exists regarding the histamine content and diamine oxidase activity in relation to human carcinomatous growths, the present investigation was undertaken and carcinomatous tissues of skin, breast and rectum were analysed. The result of the experiment gives a clear evidence that both histamine concentration and diamine-oxidase activity increase significantly in all the three types of growth. The result has been discussed and a reasonable explanation has been offered for the observations made.

Species-dependent differences of the biochemical properties of diamine oxidase

Diamine oxidase (DAO) from tissues of mice, rats and humans showed different properties with respect to stability and kinetic parameters. DAO-activities in homogenates of rat or human tissues, but not of mouse tissues, rapidly decreased upon storage at -20 degrees C. The Km-value for putrescine was 90 microM in mouse kidney or intestine. In rats different Km-values were observed before (272 microM) and after freezing (102 microM). A similar effect was observed with DAO in human kidney (321 and 39 microM, respectively). Treatment of rats with heparin resulted in a depletion of intestinal DAO and the concomitant appearance of DAO in blood. The enzyme remaining in the intestine showed the lower Km-value.

Elevation of monoacetylated polyamines in human breast cancers

Normal tissues contain only trace amounts of monoacetylated polyamines. N1-Acetylspermidine is present in high concentrations in mouse liver cells damaged by hepatotoxins and is also found in specialised cells of the hamster epididymis. In the present study human breast cancers were analysed for the presence of monoacetylated polyamines because N1-acetylspermidine is selectively elevated in human colorectal cancers. Free and monoacetylated polyamines (N1-acetylspermidine, N8-acetylspermidine and N1-acetylspermine), measured by high-performance liquid chromatography, were expressed as nmol/g wet wt of breast cancers (n = 54) or normal breast tissue (n = 15). Putrescine and monoacetylated polyamines were absent from normal breast tissue. Mean total content of monoacetylated polyamines in breast cancers 14.9 +/- 5.3 (S.E.) exceeded the mean total content of free polyamines (8.3 +/- 1.0) in normal breast tissue. Detectable levels of at least two of the monoacetylated polyamines were found in all breast cancers: N1-acetylspermidine was present in 51 (13.1 +/- 6.3), N8-acetylspermidine in 32 (0.6 +/- 0.1) and N1-acetylspermine in 28 tumours (1.2 +/- 0.3). There was no correlation between monoacetylated polyamine content of breast cancers and factors known to affect survival, i.e. tumour size, histological grade, oestrogen receptor status and node status. Monoacetylated polyamines are present in human breast cancers but not in normal breast tissue, implying that polyamine catabolism in breast cancers differs from that in normal breast tissue.

Amphicrine--composite calcitonin and mucin-producing--carcinoma of the thyroid

A case of thyroid carcinoma in a 38-year-old male with a high serum level of calcitonin is reported. The tumor was composed of polygonal argyrophilic cells, signet ring Alcian blue-positive cells, and a minority of amphicrine elements. The same polymorphic cellular component was seen at the ultrastructural level. Anticalcitonin immunoperoxidase revealed that the majority of cells were positive, including the amphicrine cells. In spite of calcitonin production it is felt that cases similar to the present one should be distinguished from "ordinary medullary" carcinoma and the terms "amphicrine" or "composite calcitonin and mucin-producing carcinoma" are proposed.

Gastrin-releasing peptide immunoreactivity in medullary thyroid carcinoma

Two cases of gastrin releasing peptide (GRP)-producing medullary thyroid carcinoma are presented. Immunohistochemical examination revealed the presence of GRP-like immunoreactivity (IR-GRP) in the primary tumor tissues. High concentration of IR-GRP was also demonstrated in extracts of the primary tumors by radioimmunologic means with use of a GRP-specific antiserum. Chromatographic analysis showed that the immunoreactivity was composed of at least two molecular forms: one behaved as synthetic porcine GRP on Sephadex G-50 gel filtration and the other as porcine GRP (14-27), a C-terminal active fragment of GRP. The IR-GRP was shown not to be attributed to bombesin-like immunoreactivity. Substance P-like immunoreactivity was not detected in the tumor tissues by either immunohistochemical or radioimmunologic means. This is, as far as the authors are aware, the first finding of IR-GRP as an ectopic product in medullary carcinoma.

Plasma histaminase activity in carcinoma of the cervix--its clinical significance

Plasma histaminase activity has been estimated in 79 patients with squamous cell carcinoma of the cervix. The mean plasma histaminase value is significantly higher in patients than in controls (P less than 0.001). The plasma histaminase activity falls significantly following radiotherapy (P less than 0.001). Pretreatment plasma histaminase values do not show any correlation with clinical stages and the posttreatment values decline significantly but remain higher in clinical stages III and IV than in stages I and II. The increased plasma histaminase activity also reflects the degree of differentiation within the tumor. Further it is noted that the plasma histaminase value is inversely proportional to the radiation response. It is suggested that estimation of plasma histaminase activity can serve as an important marker in assessing the treatment response and monitoring the course of treatment.

Histamine and calcitonin release from medullary thyroid carcinoma

The secretagogue effect of histamine on calcitonin secretion has been studied in 15 patients with medullary thyroid carcinoma and compared with known stimuli: glucagon and calcium in combination with pentagastrin. The effect of concomitant histamine H2-receptor blockade on these responses has been studied in the same patients. Seven patients with undetectable basal plasma calcitonin concentrations had measurable responses to calcium/pentagastrin but not to histamine or glucagon. In the remaining eight subjects, significant responses were seen to all three test substances, calcium/pentagastrin proving to be the most potent secretagogue. Establishment of H2-receptor blockade with cimetidine had no effect on basal calcitonin concentrations and did not suppress responses to histamine, calcium or pentagastrin. The variable secretagogue effect of histamine could be mediated through H1-receptors, through nonspecific vascular dilation "washing out" preformed calcitonin, or through its destruction to varying degrees by histaminase, present in most medullary thyroid tumors. Histamine is unlikely to replace calcium/pentagastrin as the most discriminative, provocative diagnostic agent in medullary thyroid carcinoma, but correlation of secretory responses with tissue histaminase concentrations and attempted blockade with differing antihistamines will further our understanding of this tumor.

Inhibition of lymphocyte DNA-synthetic responses by spermine-derived polycations

The polyamines putrescine, spermidine, and spermine are oxidized by the enzyme diamine oxidase to form the corresponding aldehyde derivatives. These aldehydes have been shown to undergo a variety of spontaneous reactions, some of which result in polycationic addition compounds. We have chemically synthesized some spermine-derived polycations by reaction with the dialdehyde glutaraldehyde followed by reduction of te resulting Schiff base with sodium borohydride. Their migration on ion exchange and gel filtration columns was consistent with the formation of polycations with properties similar to those reported for the spontaneous reaction products. When added to cultures of alloantigen or mitogen stimulated lymphocytes, these polycations were potent inhibitors of the incorporation of tritiated thymidine and blast cell formation. This inhibition was reversible, non-cytotoxic, and only apparent if the polycation was added early in the culture period. The concentration of polycation necessary to achieve 50% inhibition of the lymphocyte response decreased as the cationic nature relative to spermine increased.

Melanin production in a medullary thyroid carcinoma

Melanin production by a medullary thyroid carcinoma (MTC) is reported and discussed. Immunohistochemically, the tumor cells contained calcitonin; by electron microscopy, they bore numerous, heterogeneous granules similar to those described previously in MTCs. One small focus of tumor was pigmented. Here, melanosomes in different stages of maturation were found in dendritic cells that ramified among granule-bearing cells. The remarkable phenotypic divergence in this solitary, nongerm cell neoplasm is unusual but not so surprising in light of the APUD nature and neural crest origin of both the melanocyte and the thyroid C cell, which gives rise to MTC. The authors view the calcitonin and melanosome phenotypes as closely related tumor clones evolving from a common precursor neoplastic cell. This unique "experiment of nature" adds to the set of rare human tumors that make melanin "ectopically."

Cimetidine inhibits the pentagastrin-induced release of calcitonin in normocalcaemic man

Pentagastrin stimulates the release of calcitonin from normal C-cells in the human thyroid. In the present investigation the effect of cimetidine on the liberation of calcitonin in response to intraarterial pentagastrin (0.6 micrograms . kg-1) was studied in 14 normocalcaemic patients undergoing surgery for thyroid adenomas. Cimetidine was administered as a bolus injection of 200 mg followed by an intravenous infusion of 1.5 mg . kg-1 . h-1. In seven patients not given cimetidine, mean calcitonin concentration in the thyroid vein rose from 419 +/- 58 to 2787 +/- 645 pM in response to pentagastrin. In seven patients given cimetidine, mean calcitonin concentration only increased from 107 +/- 33 to 166 +/- 51 pM after pentagastrin. The difference between the two groups was statistically significant both during basal conditions (P less than 0.001) and in response to pentagastrin (P less than 0.01). The results suggest that pentagastrin affects normal C-cells via release of histamine and that cimetidine markedly interferes with this mechanism.

Histologic study of the ocular lesions in multiple endocrine neoplasia syndrome type IIb

We studied the clinical and histopathologic findings in the eyes of two patients with multiple endocrine neoplasia type IIb. A prominent clinical feature of this condition is enlarged nonmyelinated corneal nerves composed of bundles of axons associated with Schwann cells. Prominent corneal nerves within an otherwise normal corneal stroma are an important clinical feature of multiple endocrine neoplasia type IIb. Because of this, ophthalmologists are in a position to make an early diagnosis and hence to recommend diagnostic procedures and treatment for the associated potentially fatal medullary carcinoma of the thyroid and pheochromocytomas.

Diamine oxidase activity and gamma-aminobutyric acid formation in medullary carcinoma of the thyroid

The formation of different metabolites from 14C-putrescine via oxidative deamination has been studied in patients with medullary carcinoma of the thyroid (MCT) and in control subjects. The neoplastic tissue from MCT patients produced in addition to delta 1-pyrroline, large quantities of GABA and some unidentified compound(s), whereas the formation of these metabolites in normal thyroid tissue was almost negligible. Plasma from patients with MCT contained higher diamine oxidase activity than plasma from control subjects, but there was a considerable overlap between the two groups. Determination of the as yet unidentified compound(s) could possibly become a sensitive tool in the diagnosis and follow-up of carcinomas associated with a high diamine oxidase activity.

Putrescine catabolism in rats given heparin or aminoguanidine

Rats treated acutely with aminoguanidine, a potent inhibitor of diamine oxidase, or with heparin display reduced ability to metabolize 14C-putrescine to radioactive carbon dioxide. After either drug rats recover 50% of the ability to catabolize putrescine in 15--18 h. This is in close agreement with the half-time for recovery of diamine exidase activity, and indicates that putrecine-catabolizing ability of the rat reflects in a physiologically significant way the function of diamine oxidase in vivo.

Tumor cell origin of histaminase activity in ascites fluid from patients with ovarian carcinoma

We have demonstrated by tissue assay and immunohistochemical techniques that tumor cells appear to be the source of the increased histaminase activity found in 60% of ascitic fluids from patients with ovarian carcinoma. The histaminase activity in 64% of tumor tissues studied was greater than 100 U/g of tissue, a value higher than that for normal tissues other than kidney and intestine. The presence of elevated ascitic fluid histaminase activity (greater than 3.5 U/ml) correlates with the presence of high histaminase activity in tumor tissue. Histaminase activity is higher in ascitic fluid from the patients than it is in their plasma, probably reflecting the direct entry of histaminase into the peritoneal cavity.

Optimized preparation and determination of pea seedling diamine oxidase

The level of diamine oxidase in pea seedling stems has been determined as a function of time after germination in both etiolated and non-etiolated plants. The maximum amount of enzyme per plant is obtained between 11 and 13 days. The amount of activity per gram of tissue appears to be proportional to the rate of growth. We describe an efficient method of isolation of pea seedling stem diamine oxidase from 12-day-old etiolated seedlings, a procedure that brings the enzyme to purity after a 97-fold purification. A new assay procedure for pea seedling diamine oxidase is detailed and compared to previously used methods. The kinetic parameters for three common substrates have also been determined. SDS-acrylamide gel electrophoresis, gel filtration chromatography and copper analyses have been used to determine that pea seedling diamine oxidase exists as a dimer of two apparently identical subunits, the dimer molecular weight being about 190,000. The isoelectric point of this enzyme was determined to be 6.5.

Inhibition of histamine methylation in vivo by the dimaprit analog, SKF Compound 91488

The Dimaprit analog, S-[4-(N,N-dimethylamino)-butyl]isothiurea (SKF Compound 91488), which is a noncompetitive inhibitor (Ki, 0.9 microM) of histamine-N-methyltransferase in vitro, inhibited the methylation of labeled histamine in mice. When administered in combination with the diamine oxidase inhibitor, aminoguanidine, an almost complete block of histamine metabolism was observed. The inhibition was dependent on dose of drug and was apparent with doses of 50 mg/kg and greater. Since Compound 91488 is reported to have no agonist activity of its own, the drug may help to identify the actions of histamine released from endogenous stores in response to physiological and pathological stimuli.

Effect of metastatic cancer on platelet monoamine oxidase activity and serotonin metabolism

Platelet monoamine oxidase (MAO) activity and urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA), serotonin, tyramine and tryptamine were measured in patients with a variety of metastatic cancers and in healthy control subjects. The patients with cancer had higher platelet MAO activity and lower urinary excretion of 5-HIAA, sertonin and tyramine than the healthy subjects. The urinary tryptamine excretion was not significantly different in the two groups. At the present time it is unclear if the alterations in platelet MAO activity and monoamine excretion are casually related.

Levels of histaminase and L-DOPA decarboxylase activity in the transition from C-cell hyperplasia to familial medullary thyroid carcinoma

The distribution of calcitonin, L-dopa decarboxylase, and histaminase is studied in sectioned total thyroid glands from patients with different stages of familial medullary thyroid carcinoma. In 5 glands with gross carcinoma and in 3 with early microscopic carcinoma the distribution of all three parameters positively correlates (p less than .01 for each correlation). In contrast, in 6 glands with C-cell hyperplasia only the distribution of calcitonin and L-dopa decarboxylase correlates (r = 0.64, p less than .01) while those for histaminase vs. calcitonin (r = .17, p = N.S.) and histaminase vs. dopa decarboxylase (r = .03, p = N.S.) do not. In the glands with microscopic carcinoma the peak levels of histaminase occur in the areas of disease as defined by immunohistochemical staining of calcitonin; mean histaminase activity is the only one of the three parameters measured that distinguishes between C-cell hyperplasia and microscopic carcinoma (p less than .005). Immunohistochemical staining of histaminase shows positive cells in glands with gross and microscopic carcinoma, but in none of the glands with hyperplasia alone. Histaminase is thus found in high amounts in some malignant C-cells only and may be a useful marker to distinguish between hyperplasia and malignancy in thyroids with early C-cell proliferative disorders.