Diagnosis of primary thyrotrophin-secreting microadenoma by 1.5 T MR

DIAGNOSIS OF ENDOCRINE DISEASE: Diagnostic approach to TSH-producing pituitary adenoma

Thyrotropin (TSH)-secreting adenomas (TSHomas) are the rarest form of pituitary adenomas, and most endocrinologists will see few cases in a lifetime, if any. In most cases, the diagnostic approach is complicated and cases may be referred after being presented as a syndrome of inappropriate TSH secretion or as a pituitary mass. This review aims to cover the past, present and possible future diagnostic approaches to TSHomas, including different clinical presentations, laboratory assessment and imaging advances. The differential diagnoses will be discussed, as well as possible coexisting disorders. By evaluating the existing reports and reviews describing this rare condition, this review aims to present a clinically practical suggestion on the diagnosic workup for TSHomas, Major advances and scientific breakthroughs in the imaging area in recent years, facilitating diagnosis of TSHomas, support the belief that future progress within the imaging field will play an important role in providing methods for a more efficient diagnosis of this rare condition.

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.

Corticotropin- and thyrotropin-secreting pituitary microadenomas: detection by dynamic magnetic resonance imaging

Hormone-secreting pituitary microadenomas are often not visible on magnetic resonance imaging (MRI). Diagnosis requires confirmatory endocrine test results and often an invasive procedure, inferior petrosal sinus sampling (IPSS). Improved pituitary imaging may eliminate the need for IPSS in some patients, as shown in the 2 women in this report. The first patient with hirsutism, weight gain, and hypertension had intermittent elevations of urinary free cortisol, abnormal results on a low-dose dexamethasone suppression test, and positive results on a dexamethasone-suppressed ovine corticotropin-releasing hormone test (corticotropin, increase of 122%; cortisol, increase of 118%). Gadolinium-enhanced MRI showed no focal lesion, but dynamic MRI (sequential images beginning immediately after contrast injection) revealed a right-sided 5-mm microadenoma, confirmed by transsphenoidal surgery. The second patient had a goiter, anxiety, increased free thyroxine and triiodothyronine levels, and a normal thyrotropin value with no response to thyrotropin-releasing hormone. Magnetic resonance imaging showed no lesion, but dynamic MRI detected an 8-mm microadenoma. Although about 8% to 10% of healthy persons have incidental pituitary lesions that are 3 mm or larger on MRI, identification of a distinct lesion and positive results on a dexamethasone-suppressed ovine corticotropin-releasing hormone test should decrease the probability of a false-positive result on an imaging study. We recommend that dynamic MRI be performed in any patient with a suspected microadenoma, before IPSS is performed.

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.