Hyperthyroidism due to a thyrotrophin-secreting microadenoma

A pituitary tumor in a patient with thyroid hormone resistance: a diagnostic dilemma

Resistance to thyroid hormone (RTH) is due to mutations in the beta-isoform of the thyroid hormone receptor (TR-beta). RTH patients display inappropriate secretion of thyrotropin-releasing hormone (TRH) from the hypothalamus and thyrotropin (TSH) from the anterior pituitary, despite elevated levels of thyroid hormone thyroxine (T4) and triiodothyronine (T3). Thyrotropin-secreting tumors are presumed to represent clonal expansion of abnormal cells. Because the diagnosis of TSH-secreting tumors tends to be delayed and curative surgical resection remains under 50%, early diagnosis is paramount. Current diagnostic strategies suggest that RTH patients are distinguishable from patients with TSH-secreting pituitary tumors by the use of standard laboratory tests and imaging. Here, we present a woman in whom the standard evaluation for inappropriate TSH secretion was insufficient to distinguish these entities. The patient had a low-normal TRH stimulation test and an unmeasurable alpha-glycoprotein subunit level; however, a pituitary magnetic resonance imaging (MRI) revealed an adenoma. More testing using a T3 suppression test supported a RTH diagnosis and a R438H mutation was found in the TR-beta gene. To our knowledge, this represents the first report of an apparently incidental pituitary adenoma in the setting of documented resistance to thyroid hormone. As such, it raises the question of whether RTH predisposes to pituitary hyperplasia and adenoma development.

Reversible pituitary enlargement in the syndrome of resistance to thyroid hormone

We report pituitary enlargement after radioiodine ablation in a patient with elevated thyroid hormones and features of hyperthyroidism. Serum thyrotropin (TSH) levels were elevated despite normal circulating thyroid hormones, suggesting inappropriate TSH secretion associated either with a TSH secreting pituitary adenoma or resistance to thyroid hormone (RTH). Normal serum glycoprotein alpha-subunit levels and a preserved TSH response to thyrotropin-releasing hormone (TRH) favored RTH and this diagnosis was confirmed by showing the patient to be heterozygous for a missense mutation (R438H) in the thyroid hormone beta receptor (TRbeta) gene. Thyroxine replacement in supraphysiological doses were required to normalize TSH levels and resulted in regression of the pituitary enlargement, suggesting hyperplasia rather than coincident tumor. This case illustrates the need to avoid thyroid ablation in RTH patients and the importance of supraphysiological thyroxine replacement to prevent pituitary hyperplasia.

A case of thyrotropin-secreting pituitary microadenoma with normal thyrotropin alpha-subunit level

We present a 32-year-old male with a thyrotropin (TSH)-secreting pituitary microadenoma with normal alpha-subunit (SU) and/or alpha-SU/TSH molar ratio. An interesting feature of this patient is that the size of the pituitary tumor remained unchanged during a 6-year follow-up without treatment. The tumor was clearly visualized with somatostatin receptor imaging, indicating that it was somatostatin receptor-positive. Subcutaneous injection of 100 microg octreotide acetate three times daily resulted in significant reduction of TSH and free thyroid hormones 6 weeks after initiation of treatment. However, tumor size was not changed 3 months after initiation of octreotide therapy and thyroid hormones, but not TSH level, eventually increased in spite of increasing the octreotide dosage up to 600 microg/day. This led to discontinuation of treatment. The patient responded only temporarily to octreotide in spite of somatostatin receptors. This case further demonstrates that a normal alpha-SU and/or alpha-SU/TSH molar ratio cannot exclude the possibility of a TSH-secreting pituitary adenoma.

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.

Hyperthyroidism caused by an ectopic TSH-secreting pituitary tumor

A woman developed what appeared to be typical Graves' disease in 1965 at the age of 45 years. After 9 years of antithyroid drug therapy, she was treated with radioiodine. Ten years later (1985) she developed postablative hypothyroidism. Despite replacement doses of thyroxine that resulted in thyroid hormone levels that were in the hyperthyroid range, TSH levels remained elevated. Initial biochemical studies, including a high alpha-subunit to TSH ratio, suggested a pituitary TSH-secreting tumor, but a CT scan of the sella turcica was normal. In 1994, while undergoing an otolaryngologic examination, the patient was found to have a nasopharyngeal mass lesion, which was ultimately shown histologically and immunohistochemically to be an ectopic pituitary tumor. Resection of the mass restored TSH and alpha-subunit levels to normal. This patient probably represents the first ectopic TSH-secreting pituitary tumor to be reported.

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.

Clinical hyperthyroidism due to non-neoplastic inappropriate thyrotrophin secretion

We report a case of hyperthyroidism due to inappropriate thyrotrophin (TSH) secretion in a patient with selective pituitary resistance to thyroid hormone action. Symptoms of hyperthyroidism in patients with this disorder are usually mild, implying some peripheral tissue resistance to the metabolic effects of thyroid hormone. Our patient had unusually severe symptoms, including marked weight loss and cardiac arrythmias which required carbimazole and beta-blocker therapy for control. Somatostatin was ineffective in suppressing TSH secretion. The introduction of sensitive thyrotrophin assays should facilitate the accurate diagnosis of TSH-induced hyperthyroidism and avoid inappropriate treatment.

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.

Effects of thyroid hormones (T4,T3), bromocriptine and Triac on inappropriate TSH hypersecretion

Inappropriate TSH hypersecretion was diagnosed in a 38-year-old woman (case 1) and in a 38-year-old man (case 2). Both of them had earlier been treated by ablative therapy for hyperthyroidism. The present diagnosis was based on elevated basal serum TSH levels despite elevated serum free thyroid hormone levels. Both of them had exaggerated TSH responses to TRH (peak value 240 mU/l in case 1 and 408 mU/l in case 2). Their albumin and prealbumin levels were normal. The serum TBG level was normal in case 1 but was elevated in case 2. Serum levels of alpha-subunits of TSH, and pituitary CT scans were normal. Despite mild clinical hyperthyroidism, peripheral indices of thyroid hormone action were normal. They had also relatives with apparent resistance to thyroid hormones. In view of the possibility that prolonged pituitary thyrotrophic stimulation is detrimental, various therapeutic approaches to suppress TSH levels were tried. Both T3 and T4 treatments lowered serum TSH levels, but were poorly tolerated. Acute administration of L-dopa or bromocriptine reduced serum TSH levels, but this was not seen during long-term therapy. TRIAC treatment lowered serum TSH levels, and the drug was well tolerated. Serum TSH responses to TRH were not blunted during T3, T4 or TRIAC treatments. Somatostatin also reduced serum TSH levels, but did not potentiate the effect of low dose T3 therapy. Our results suggest that the patients had unbalanced pituitary and peripheral thyroid hormone resistance, predominantly at the pituitary level. Of the drugs studied, TRIAC seemed to be the most suitable therapy.

Medical management of pituitary disease

Success in the treatment of pituitary tumours depends on their size and invasiveness so that early detection is imperative. Immediate cure of microadenomas is assured by trans-sphenoidal microsurgery although longterm results are not yet established. Complete removal of large tumours is less likely and radiotherapy is advised if there is evidence of residual tumour activity. External irradiation is indicated in the treatment of radiosensitive germinomas. Drugs can control symptoms before surgery or while awaiting the effects of radiotherapy but may be the treatment of choice for large prolactinomas. Bromocriptine or other dopamine agonists not only inhibit prolactin secretion but also limit tumour size, but it is not known if they are effective indefinitely. Dynamic tests of pituitary function should precede hormone replacement therapy and careful supervision with monitoring of plasma hormone levels accompany long-term treatment.