A 32-year-old woman was noted to be tachycardic during elective surgery. Thyroid function was abnormal and she was referred to endocrine team. She gave 10 year history of galactorrhoea, palpitations , tremor and diaphoresis with frontal headaches. There were no clinical features suggestive of Acromegaly. Heart rate was 100 beats /minute and regular. There was no visual field deficit or clinical features of acromegaly.
|
Investigation |
Initial |
Repeat |
Reference range |
|
TSH |
2.8 |
1.94 |
0.5 - 4.5 mIU/L |
|
FT4 |
20.6 |
25.8 |
10-20 pmol/L |
|
FT3 |
8.8 |
8.3 |
3.1-5.4 pmol/L |
|
Prolactin |
|
936 |
90 - 630 mIU/L |
|
IGF 1 |
|
51 |
9-33 nmol/L |
|
SHBG |
|
83 |
25-90 nmol/L |
Assay interference was excluded as a cause for the thyroid dysfunction.
The differential diagnosis was TSH producing pituitary adenoma versus resistance to thyroid hormone ( RTH) .She then underwent a TRH stimulation test with results consistent with a diagnosis of a TSH secreting macroadenoma . Post TRH administration, TSH did not show a 2-fold rise, with a pre-TRH level of 2.38 mIU/L and post-TRH level of 2.31 mIU/L at 20 minutes and 1.88 mIU/L at 60 minutes.
|
Time (min) |
TSH (mlU/L) |
Free T4 (pmol/L) |
|
Baseline |
2.38 |
38 |
|
0 |
IV bolus of 200 ug of TRH |
|
|
20 |
2.31 |
4 |
|
60 |
1.88 |
37 |
Pituitary MRI showed a large cell mass with suprasellar extension measuring 1.7 x 1.6 x 2 cm approaching optic chiasm. Normal pituitary gland could not be identified as a separate structure.
TSHomas are a rare cause of hyperthyroidism and account for less than 2% of all pituitary adenomas . Most TSH-secreting adenomas secrete only TSH. Approximately 20 to 25 percent of the adenomas secrete predominantly growth hormone or prolactin. Hyperprolactinemia is not always due to tumour secretion of prolactin but can be caused by compression of the pituitary stalk with interruption of hypothalamic inhibition of prolactin secretion.