Pituitary adenomas with hyperfunction of TSH. Frequency, histological classification, immunocytochemistry and ultrastructure

[The 2017 WHO classification of pituitary tumors]

The 2017 WHO classification of pituitary tumors is still based on structural analyses and expression of various pituitary hormones. Three innovations have to be considered: (1) The expression of pituitary transcription factors Pit‑1, T‑Pit and SF‑1. (2) The term "atypical adenoma" was replaced by "aggressive adenoma". (3) The three tumor types of the neurohypophysis (pituicytoma, spindle cell oncocytoma, granular cell tumor) are defined by their common expression of TTF‑1. Craniophyryngiomas are identified as adamantinomatous type by focal nuclear expression of β‑catenin or as papillary type by demonstration of BRAF V600E mutation. Further primary tumors of the pituitary are extremely rare. These and also the other tumors of the sellar region can be structurally very similar to pituitary adenomas but can be-nearly without exception-differentiated by immunocytochemistry.

Clinical Impact of the Current WHO Classification of Pituitary Adenomas

WHO classifications should be used for comparing the results from different groups of pathologist and clinicians by standardized histopathological methods. Our present report describes the important parameters of pituitary adenoma pathology as demand of the WHO classification for correlation to endocrine data and prognosis. The combination of HE stain based structures with immunostainings for pituitary hormones allows subclassification of adenomas as the best method not only for correlations to clinical hyperfunctions but also for statements to the sensitivity of drug therapies (somatostatin analogs, dopamine agonists). GH-, PRL- and ACTH-secreting pituitary adenomas are further classified based on the size and number of their secretory granules by electron microscopy, or as is mostly the case nowadays by cytokeratin staining pattern, into densely and sparsely granulated. Granulation pattern may be considered for the prediction of treatment response in patients with GH-secreting adenomas, since the sparsely granulated subtype was shown to be less responsive to somatostatin analog treatment. For prognosis, it is important to identify aggressive adenomas by measurements of the Ki-67 index, of the number of mitoses, and of nuclear expression of p53. Among the criteria for atypical adenomas, high Ki-67 labeling index and invasive character are the most important adverse prognostic factors. Promising molecular markers have been identified that might supplement the currently used proliferation parameters. For defining atypical adenomas in a future histopathological classification system, we propose to provide the proliferative potential and the invasive character separately.

Thyroid storm induced by TSH-secreting pituitary adenoma: a case report

Thyroid stimulating hormone-secreting pituitary adenomas (TSHomas) are uncommon tumors of the anterior pituitary gland. Patients with TSHomas may present with hyperthyroidism, but the incidence of thyroid storm due to TSHomas has yet to be determined. We report a rare case of thyroid storm caused by TSHoma in a 54-year-old woman. Preoperatively she had symptoms of excessive sweating and palpitation. Blood tests showed inappropriate secretion of TSH with blood TSH 6.86 μ U/mL, fT3 19.8 pg/mL, and fT4 5.95 ng/dL. Magnetic resonance imaging (MRI) revealed a pituitary tumor with maximum diameter of 13 mm that was extirpated through transsphenoidal route. After operation the patient was stuporous and thyroid storm occurred presenting with hyperthermia, hypertension, and tachycardia. It was well managed with nicardipine, midazolam, steroids, and potassium iodide. Immunohistochemical staining of tumor specimen was positive for TSH and growth hormone (GH). One year after operation, fT3 and fT4 levels were still high. As her tumor was diagnosed to be GH- and TSH-producing adenoma, octreotide injection therapy was started, which normalized thyroid hormone levels. This is the second reported case with thyroid storm due to TSHoma and emphasizes the importance of strategies with interdisciplinary cooperation for prevention of such emergency conditions.

Expression of type 5 somatostatin receptor in TSH-secreting pituitary adenomas: a possible marker for predicting long-term response to octreotide therapy

In TSH-secreting pituitary adenomas (TSHoma), octreotide (OCT) therapy reduces tumor size and TSH secretion in some cases but not in others. As OCT acts through various types of somatostatin receptors (SSTRs), the different responses of TSHoma to OCT might be explained by the differences of SSTR expression. We therefore studied the expression of subtype-specific SSTR mRNA transcripts in tumor tissues by RT-PCR. Type 2 (SSTR2) mRNA transcripts were detected in all 8 tumors but those of SSTR3 and SSTR5 were demonstrated only in 5 of them. Serum TSH levels were decreased by OCT administration test in all patients but OCT therapy was effective in two patients out of three. SSTR5 mRNA was detected in two tumors from the responder, but not in one tumor that was resistant to OCT. These observations suggest that the temporal decrease of TSH by OCT may be mediated by SSTR2, and that the long term response to OCT therapy may be related with the expression of SSTR5. Therefore, the expression of SSTR5 in TSHoma may be a useful marker for predicting the outcome of the therapy, but further studies with larger numbers of patients are necessary.

[Pituitary gland tumors]

Pituitary adenomas must be clearly differentiated from other tumors of the sellar region (especially meningiomas, granular cell tumors, chordomas and germinomas), which may look very similar. The sub-classification of adenomas depends on the methods used, in particular the immunostaining for pituitary hormones. This sub-classification is not necessary in every case, but must be performed if unusual findings are observed during surgery or if surgery is unsuccessful and radiation or drug-therapy is planned. Special structures and non-immunohistochemical stainings are very helpful for typing adenomas. We differentiated monohormonal densely or sparsely granulated GH-cell adenomas, monohormonal sparsely or very rarely densely granulated prolactin cell adenomas, monohormonal densely or sparsely ACTH-cell adenomas, monohormonal TSH-cell adenomas and FSH/LH cell adenomas from bihormonal adenomas of mammosomatotroph or GH/prolactin cell type or of the acidophil stem cell adenoma type. The number of plurihormonal adenomas decreased with the use of improved monoclonal antibodies. Clinically inactive adenomas are classified as null cell adenomas, oncocytic adenomas or FSH/LH-cell adenomas. These appear as subtypes of one entity deriving from the gonadotroph cell type. Craniopharyngiomas are classified into adamantinous and papillary types, which are not only structurally but also clinically different. If adamantinous craniopharyngiomas show very strongly regressive changes, immunostaining for keratin may be necessary to identify the squamous epithelia for the demonstration of craniopharyngioma.

Rheumatic manifestations of pituitary tumors

Pituitary tumors may cause rheumatologic problems as a result of under production or overproduction of one pituitary hormone. Excessive growth hormone causes destruction of cartilage by a direct action. Facial and acral changes and arthralgias may be some of the first symptoms of acromegaly. The arthritis associated with acromegaly is often devastating. Carpal tunnel syndrome is very common in patients with acromegaly. Adrenocorticotropin (ACTH) has indirect effects via the action of glucocorticoid on bones, muscles, and the immune system. Proximal muscle weakness is a characteristic feature of Cushing's syndrome. Patients with Cushing's syndrome commonly have osteopenia and osteoporosis that lead to an increase in bone fractures. Avascular necrosis is associated with exogenous steroid administration. The effects of too much glucocorticoid or too rapid withdrawal can be severe. Gonadotropins act via the gonadal steroids and protect bone mass from loss. Prolactin is less involved in rheumatologic disease; the data for which are limited in humans. Pituitary tumors can have manifestations similar to rheumatologic disorders and should be included in the differential diagnosis of these diseases.

Ultrastructural characteristics of TSH-producing adenomas with special reference to its close similarity to BFA-treated pituitary adenoma cells

Two of 420 patients with pituitary adenoma who underwent operation from 1989 to 1997 had thyroid stimulating hormone (TSH) producing adenoma. We investigated these TSH cell adenomas with immunohistochemical and ultrastructural methods and compared their ultrastructural features with brefeldin A (BFA, 0.5 mg/ml) treated pituitary adenoma cells. BFA-treated pituitary adenomas include a prolactin (PRL) cell adenoma, a growth hormone (GH) cell adenoma, an adrenocorticotropic hormone (ACTH) cell adenoma, a gonadotroph adenoma, and a plurihormonal adenoma. Immunohistochemical staining disclosed that one of the TSH cell adenomas produced only TSH-beta and that another produces both TSH-beta and FSH-beta. Ultrastructural analysis showed the abundance of oval-shaped dilated rough endoplasmic reticulum (rER). Within the dilated rER, the mistlike deposit or deposit along the inner margin of the rER membrane was observed. On the other hand, BFA-treated cultured pituitary adenoma cells showed the opening of the cavity of the rER cisterna and they enlarged to an oval form with time and revealed an accumulation of electron-dense deposits within the dilated rER. These ultrastructural similarities between TSH cell adenoma and BFA-treated pituitary adenoma cells indicate the functional disturbances in the secretory passage through the Golgi apparatus in TSH cell adenoma cells.

Difficulty in differentiating thyrotropin secreting pituitary microadenoma from pituitary-selective thyroid hormone resistance accompanied by pituitary incidentaloma

A 33-year-old woman with inappropriate secretion of TSH and a 2-mm pituitary microadenoma is described. She had a high serum free T4 concentration (31 pmol/L) with an inappropriately nonsuppressible serum TSH concentration (0.93 mU/L). The alpha/TSH molar ratio was 2.3 and magnetic resonance imaging with gadolinium enhancement identified an area of low signal intensity in the left lateral pituitary gland. However, TSH secretion was not completely autonomous. There was a significant response to exogenous TRH stimulation and suppression by T3 administration. Therefore, it was difficult to rule out a nonfunctioning pituitary adenoma with concomitant pituitary selective thyroid hormone resistance syndrome. A 2-mm microadenoma was excised via transsphenoidal surgery. The tumor cells were immunoreactive to antisera to alpha-subunit and minimally immunoreactive to antisera to TSHbeta. The patient's thyroid function normalized after surgery without medication. Because the adenoma could become large and intractable if the patient was treated inadequately, early diagnosis and treatment are important in patients with TSH secreting adenomas.

Thyrotropin-producing pituitary adenomas

To evaluate the biology of thyrotropin (TSH)-producing pituitary adenomas, the authors reviewed the charts of 19 patients who underwent transsphenoidal surgery within a 15-year period at the University of California, San Francisco (UCSF). Between 1989 and 1991, the period during which immunostaining techniques were used consistently for diagnosis, 2.8% of the pituitary adenomas treated at UCSF were TSH-producing. The rate of reoperation for tumor recurrence was 10.5%. Before pituitary surgery, more than one-third of the 19 patients had undergone thyroid ablation. Two patients had a history of Hashimoto's thyroiditis. The female:male ratio was 1.7:1. Women tended to develop these tumors at a younger age and had a longer history of symptoms but their tumors were smaller and less often invasive than those seen in men. About 50% of the tumors were purely TSH-producing and 50% were plurihormonal, including five that produced both TSH and adrenocorticotroph hormone. All tumors were macroadenomas. Before surgery, 46% of the patients had abnormal electrocardiographic findings; 16% had a rapid onset of severe neurological conditions either before or after surgery. It is concluded that TSH-producing adenomas are more common in patients who undergo surgical treatment than was previously thought. In addition, they occur more frequently in women, have a different biology in women than in men, and tend to be associated with potentially life-threatening cardiovascular and neurological complications.

Ultrastructural diagnosis of human pituitary adenomas

Electron microscopy, which has been instrumental in the characterization of normal pituitary cell types, has also played a crucial role in the morphologic classification of pituitary adenomas arising in the presently known 5 cell types, and in the recognition of 3 adenoma types with yet undisclosed cell derivation. This review deals with the application of electron microscopy for study of pituitary adenomas in order to provide specific pathological diagnosis and aid the clinician in selecting appropriate postoperative treatment. In addition to the ultrastructural appearance and diagnostic features of 15 adenoma types, the morphology of hyperplastic proliferations and that of known normal counterparts of various adenoma types are also discussed. Specific morphologic diagnosis of pituitary lesions is important not only for adequate postoperative management of patient, but is also a prerequisite for study of the natural history and biological behaviour of various adenoma types.

Surgical treatment of thyrotropin-secreting pituitary adenomas

Thyrotropin-secreting pituitary adenomas have been diagnosed more frequently as radiographic techniques and biochemical assays have improved; however, they remain uncommon and are unfamiliar to most neurosurgeons. This report concerns eight patients with hyperthyroidism, inappropriately elevated levels of serum thyrotropin and alpha-subunit, and radiographic evidence of pituitary tumor. All underwent surgery and had pathological confirmation of a thyrotropin-secreting adenoma, and most had been subjected to prior ablation of the thyroid gland. Only one tumor was a microadenoma; the others ranged in size from 1.4 to 12 cm, and invasion of parasellar structures was common. Thyrotropin, triiodothyronine, thyroxine, and alpha-subunit were measured preoperatively and at intervals postoperatively. Coexistent hormonal abnormalities (which occurred in all patients) included acromegaly and hyperprolactinemia and were also monitored. All four patients who had tumors less than 2 cm in diameter remain alive. Complete extirpation of tumor in these patients produced rapid correction of all hormonal abnormalities and resolution of clinical hyperthyroidism. The other four patients had larger invasive tumors: two died soon after surgery, one died of disseminated tumor 8 years after presentation, and one remains alive with residual tumor. Tumors secreting thyroid-stimulating hormone are less easily cured by surgery than are other types of pituitary adenoma because of the large size and invasive features that many attain during the delay to diagnosis; medical therapy can subdue the tumor but not cure it. The experience with these patients establishes the importance of early diagnosis and surgical excision for successful treatment, and demonstrates the utility of modern diagnostic techniques for finding these lesions. As occurs in Nelson's syndrome after adrenalectomy for Cushing's disease, ablation of the target organ may allow the tumor to convert to a more clinically malignant form which is resistant to cure.

[Pituitary adenomas: immunohistochemical study of 167 cases]

One hundred and sixty seven cases of pituitary adenoma were analysed using the immunocytochemical method of the Avidin-Biotin Complex (ABC), described by Hsu et al. (1981). Six pituitary anti-hormones were utilized: anti-prolactin (aPRL) at a 1:1,500 dilution; anti-growth hormone (aHGH) at a 1:4,000 dilution: anti-adrenocorticotrophic hormone (aACTH) at a 1:3,000 dilution; anti-thyrothrophic hormone (aTSH) at a 1:3,000 dilution; anti-luteinizing hormone (aLH) at a 1:1,000 dilution; and a anti-follicle-stimulating hormone (aFSH) at a 1:300 dilution. Incubation period was 14 to 16 hours at 4 degrees C. The survey of clinical, laboratory and radiological data of cases of pituitary adenomas was performed after reading the stained slides using the immunocytochemical method. Of the 167 cases of pituitary adenomas, 136 (81.4%) disclosed a positive immunoreaction to one or more anti-hormones, and the positivity index of neoplastic cells varied from 1 to 90%. The immunoreaction was positive exclusively to one anti-hormone in 80 cases (58.8%) and to two or more anti-hormones in 56 cases, and the association most frequently found was between both aPRL and aHGH. The positivity to the immunoreaction was distributed as follows: -100 cases were positive for aPRL, exclusively in 4 cases; -65 cases were positive for aHGH, exclusively in 22 cases; -31 cases were positive for aACTH, exclusively in 8 cases; -5 cases were positive for aTSH, exclusively in one case; -one patient presented an adenoma positive to aLH and another patient to aFSH.

Severe hyperthyroidism due to neoplastic TSH hypersecretion in an old man

A case is reported of neoplastic TSH hypersecretion in a 62-year-old man with severe hyperthyroidism and cardiovascular disease. He had been known to be hyperthyroid for 14 yr, and had been treated by thyreostatic drugs and subtotal thyroidectomy without satisfactory results. When he was referred to our Center, he was frankly hyperthyroid with both TSH (14 microU/ml) and thyroid hormone serum levels (TT4 24 micrograms/dl, TT3 370 ng/dl, FT41 7.9) above the normal range. alpha-subunit serum level was markedly increased (7.2 ng/ml), while beta-subunit was only 0.3 ng/ml. Skull X-ray showed an enlarged sella turcica with destruction of the dorsum and an intrasellar tumor was visualized on conventional and computer tomography. TSH response was absent after TRH and domperidone, while TSH serum levels decreased by 25% after bromocriptine. Methimazole therapy temporarily decreased serum thyroid hormones to normal levels, while TSH levels rose to 34 microU/ml, thus indicating that pituitary-thyroid feed-back was maintained at a higher set point. Surgical attempt failed because of cardiac problems during anesthesia. Radiotherapy plus methimazole was begun and TSH serum levels first increased markedly, up to 140 microU/ml, and then progressively decreased without reaching normal values. After methimazole withdrawal hyperthyroidism recurred.

The TSH secretion in the human pituitary adenomas

TSH secretion by a pituitary tumor is very rare (2%) and it is often associated with another hormone: GH or PRL essentially. We present here nine tumors in which the TSH secretion was proved by immunocytochemistry (ICC) and by RIA in the tumor extracts, in the serum and in the culture medium. Four tumors secreted TSH only. Five tumors secreted TSH and GH predominantly. In 3 of them traces of other hormones (PRL and FSH) were also detected. The "pure" TSH adenomas were monomorphous with typical ultrastructural and immunocytochemical features. Plurihormonal TSH adenomas were bimorphous with different cells secreting GH and TSH or monomorphous with one type of cell which secreted TSH or GH or both TSH and GH. In a majority of the cases, the tumoral TSH secretion induced hyperthyroidism but in 2 patients with TSH adenoma there was euthyroidism and in another with TSH-GH adenoma there was no sign of acromegaly and GH serum levels were normal.

Pituitary gland

This review summarizes current knowledge on pathology of proliferative lesions of the human pituitary. The morphologic classification of pituitary adenomas--based on histology, immunohistochemistry and electron microscopy--has now been firmly established. It has been conclusively proven that all presently recognized adenohypophysial cell types give rise to adenoma and all known pituitary hormones may be secreted in excess. Evidence is accumulating that hyperplasia of various adenohypophysial cell types can lead to hypersecretory syndromes similar to those associated with the corresponding adenomas. Owing to the rarity of studies on pituitary hyperplasia, the condition is still incompletely defined. The difficulties regarding morphologic diagnosis of pituitary hyperplasia are discussed. Despite major advances in the field of pituitary pathology, several problems concerning structure-function relationship, as well as pathogenesis of proliferative lesions are still unresolved. There is strong circumstantial evidence suggesting that the cytological mapping of the pituitary is incomplete and there are still cell types waiting to be discovered.

Pituitary adenomas

This review highlights various aspects of the new functional classification of pituitary adenomas which is based on detailed immunohistochemical and ultrastructural analysis and correlation with clinical and biochemical findings. In addition, current investigation of the non-hormonal aspects of these tumours is discussed, including the application of flow cytometry in tumour ploidy studies.

Pathology of excessive production of growth hormone

Since its clinical description in the last century, much progress has been made in our understanding of acromegaly. From an initial description of pituitary enlargement as just another manifestation of generalized visceromegaly, the pituitary abnormality has come to be recognized, in most instances, as the underlying aetiological factor. Gigantism and acromegaly are manifestations of disordered pituitary physiology, but the lesion responsible may be hypothalamic, adenohypophyseal or ectopic in location. The best known pathological hypothalamic basis for acromegaly is represented by a neuronal malformation or 'gangliocytoma'. It usually takes the form of an intrasellar gangliocytoma or, more rarely, a hypothalamic hamartoma. The neuronal elaboration of GHRH may play a role in the development of a growth hormone adenoma; the pituitary process may pass through an intermediate stage of somatotropic hyperplasia. When acromegaly has its basis in a pituitary abnormality, the lesion is almost exclusively an adenoma; the non-tumorous adenohypophysis shows no evidence of coexistent hyperplasia. Surprisingly, such tumours are more often engaged in the formation of multiple hormones rather than GH alone. They frequently produce not only GH and prolactin, the products characteristics of cells of the acidophil line, but also glycoprotein hormones, usually TSH. The spectrum of adenomas also varies in its degree of differentiation from a histogenetically primitive lesion, the acidophil stem cell adenoma, to well-differentiated tumours of varying cellular composition and hormone content. Each adenoma type has its clinicopathological, histochemical, immunocytological and ultrastructural characteristics. The isolation and characterization of GHRH has permitted the identification of neuroendocrine tumours, most of foregut origin, elaborating this releasing hormone. Such functional tumours induce hyperplasia of pituitary somatotrophs and may, on occasion, result in the formation of growth hormone adenomas. Resection of these GHRH-producing neoplasms results in reversal of endocrinological and sellar abnormalities. Future efforts should be directed toward the elucidation of the aetiology of pituitary adenomas, specifically whether they represent a proliferative process having its origin in endocrinological imbalance, presumably a hypothalamic abnormality, or whether it has a 'de novo' origin in the 'usual process of neoplastic transformation'.

Regulation of hormone release by cultured cells from a thyrotropin-growth hormone-secreting pituitary tumor. Direct inhibiting effects of 3,5,3'-triiodothyronine and dexamethasone on thyrotropin secretion

The regulation of TSH and GH secretion was investigated in cultured tumor cells prepared from a mixed TSH/GH secreting pituitary tumor. The tumor tissue had been removed transsphenoidally from a patient with hyperthyroidism and inappropriately high serum TSH levels and acromegaly. TSH and GH secretion by cultured cells were stimulated in a parallel way by TRH (300 nM) and LHRH (50 nM), but were unaffected by bromocriptine (10 nM). Exposure of the tumor cells to dexamethasone (0.1 microM) or T3 (50 nM) had differential effects on hormone secretion. GH secretion was greatly stimulated by dexamethasone, but unaffected by T3. TSH secretion was inhibited both by T3 and by dexamethasone. So, T3 and glucocorticoids inhibit TSH release by the human pituitary tumor cells studied at least partly by means of a direct effect.

Histological classification of pituitary disease

Morphological features of pituitary disease are classified according to increased and decreased hormone production to allow clinical correlation with pathological processes. Increased hormone synthesis and secretion may be due to pituitary adenomas or carcinomas derived from the five hormone-secreting cell types, or to extrapituitary stimuli causing hypertrophy and hyperplasia of those cells. Various tumour-like conditions can mimic functioning adenomas. Rarely, no lesion is detected and intrinsic abnormalities of adenohypophyseal cells are implicated. Hypopituitarism can be selective or generalized. Diffuse hormone deficiency is usually attributable to tissue destruction by tumours, inflammatory or infiltrative conditions or vascular lesions. Congenital abnormalities of pituitary development may result in hypophyseal dysfunction. Hypothalamic abnormalities may cause generalized hypopituitarism or may involve only selective releasing factors and hormones. Feedback inhibition and receptor abnormalities may be implicated in pituitary hypofunction, and selective deficiencies may be the result of genetic abnormalities, immune reactions or toxic damage to one cell type.

Pituitary adenomas producing growth hormone, prolactin, and one or more glycoprotein hormones: a histologic, immunohistochemical, and ultrastructural study of four surgically removed tumors

The morphologic features of four pituitary adenomas, removed from 2 men and 2 women between 31 and 62 years of age, are reported. The tumors contained growth hormone (GH), prolactin (PRL), and one or more glycoprotein hormones--usually thyrotropin (TSH). Three tumors were associated with acromegaly and one with hyperprolactinemia. Hyperthyroidism was not evident in any of the patients. In the tumors of acromegalic subjects, GH-containing cells were the most numerous, whereas PRL cells were dominant in the adenoma accompanied by hyperprolactinemia. Electron microscopy revealed plurimorphous tumors comprised of various proportions of morphologically different cell types: densely granulated GH cells, TSH-like cells, and the less common mammosomatotrophs and PRL cells. It is suggested that pituitary adenomas producing GH, PRL, and glycoprotein hormones derive from the same precursor; their immunocytochemical profile, fine structural appearance, and endocrine function may depend on the degree and direction of the cellular differentiation.

TSH-secreting pituitary tumor. A case report

A pituitary tumor secreting thyroid-stimulating hormone (TSH) only was diagnosed in a 43-year-old man who originally showed signs and symptoms of thyrotoxicosis which recurred twice. Despite normal serum levels of T3 and T4, increased levels of TSH were diagnosed. The pituitary tumor was operated by the transsphenoidal approach. Morphological analysis of tumor tissue showed cellular pleomorphism and regressive changes. Nuclear DNA analysis showed that the tumor had only one cell line and an aneuploid DNA pattern. In vitro culture of tumor tissue confirmed that the tumor secreted TSH only.

Hyperthyroidism due to a TSH secreting pituitary adenoma: case report, treatment and evidence for adenoma TSH by morphological and cell culture studies

A 36-year-old woman with recurrent hyperthyroidism, inappropriately elevated serum TSH, and an 8 mm pituitary microadenoma is described. Transsphenoidal adenomectomy rapidly reduced serum TSH to normal and restored the euthyroid state with retention of other anterior pituitary functions. Tissue removed at operation was examined by light and electron microscopy and cell culture. The tissue was neoplastic, composed of irregular often elongated cells which immunostrained positively only with antisera to beta-TSH. The cells contained small granules (100-170 nm) usually along the cell membrane. In cell culture TSH alone was secreted and the rate of secretion declined with time. We conclude that the patient had a TSH secreting microadenoma as a cause of her hyperthyroidism.