Thyroid Function Tests for Assessing Pituitary Adenoma
For assessing pituitary adenoma, a complete evaluation of all anterior pituitary hormone axes is necessary, including thyroid function tests that should measure TSH, free T4, and free T3 levels. 1
Primary Thyroid Function Tests for Pituitary Adenoma
- Measure TSH, free T4, and free T3 levels to identify potential TSH-secreting adenomas (TSHomas), which present with elevated or normal TSH with elevated free T4 and free T3 2, 3
- Routine prolactin testing should be performed in all patients with suspected pituitary adenoma, as hyperprolactinemia frequently coexists with TSHomas 1, 4
- Complete evaluation of all anterior pituitary axes is necessary as panhypopituitarism occurs in 6-29% of patients with pituitary disorders 1
Dynamic Testing for TSH-Secreting Adenomas
- T3 suppression test: In normal individuals, T3 administration suppresses TSH, while in TSHoma patients, TSH remains unsuppressed 5
- TRH stimulation test: Patients with TSHomas typically show blunted or absent TSH response to TRH stimulation 5
- These dynamic tests help distinguish TSHomas from syndromes of resistance to thyroid hormone action 5
Additional Testing for Comprehensive Assessment
- Assess for hypopituitarism by testing other pituitary hormone axes, as hypopituitarism is common in patients with pituitary macroadenomas 6
- Measure IGF-1 levels to identify potential co-secretion of growth hormone, as many TSHomas are plurihormonal 3, 7
- Test for gonadotropins (LH, FSH) and sex steroids, as hypogonadism is frequently present in patients with pituitary adenomas 1
- Assess adrenal function with cortisol testing, as secondary adrenal insufficiency may be present 4
Imaging and Follow-up Testing
- MRI with contrast is the imaging modality of choice for detailed anatomical delineation of pituitary adenomas 6
- Visual assessment, including visual acuity, visual fields, and fundoscopy, should be performed in all patients with pituitary macroadenoma 2
- Monthly thyroid function tests for 6 months after initial treatment of TSHomas are recommended for monitoring 2
Special Considerations
- Genetic assessment should be offered to all children and young people with pituitary adenomas to inform management and family surveillance 2, 1
- Immunohistochemical staining for pituitary hormones and Ki-67 should be performed on surgically resected pituitary adenoma tissue 2
- In patients with suspected TSHoma who have undergone thyroid ablation or are on thyroid hormone replacement, TSH may be inappropriately normal rather than elevated 8
Common Pitfalls and Caveats
- TSHomas are often misdiagnosed as primary hyperthyroidism, leading to inappropriate thyroid ablation which can cause further pituitary tumor expansion 8, 5
- Plurihormonal tumors are common, with 86% of TSHomas co-secreting GH and/or prolactin, requiring comprehensive hormonal evaluation 3
- Subtle changes in thyroid function tests may be initially missed, emphasizing the need for careful interpretation of TFTs over time 4
- Failure to recognize central hyperthyroidism (TSHoma) versus primary hyperthyroidism can lead to inappropriate treatment and worsening outcomes 8, 5
By following this comprehensive approach to thyroid function testing in patients with suspected pituitary adenoma, clinicians can accurately diagnose TSHomas and other pituitary disorders, leading to appropriate treatment decisions and improved patient outcomes.