In a patient with an incidental 3 mm pituitary microadenoma, normal adrenocorticotropic hormone, low‑normal morning cortisol, mildly elevated thyroid‑stimulating hormone and normal free thyroxine, what is the appropriate evaluation and management?

Management of 3mm Pituitary Microadenoma with Borderline Hormonal Findings

This 3mm pituitary microincidentaloma requires endocrine evaluation to exclude subclinical hypersecretion, dedicated pituitary MRI with contrast, and clinical follow-up, but does not warrant surgical intervention at this time. 1

Immediate Endocrine Evaluation Required

Your patient's laboratory values reveal two concerning patterns that require further investigation:

• The AM cortisol of 6.4 μg/dL is low-normal and warrants a 1mg overnight dexamethasone suppression test to exclude subclinical Cushing's syndrome, as approximately 5% of radiologically benign pituitary incidentalomas have subclinical hormone production. 1

• The TSH of 4.50 mIU/L with normal free T4 suggests possible subclinical hypothyroidism or early TSH-secreting adenoma (though the latter is rare). Repeat thyroid function testing and consider additional pituitary hormone evaluation including IGF-1 and prolactin levels. 1

• Complete the hormonal workup with: 1

  • Prolactin level (to exclude microprolactinoma, the most common functional microadenoma)
  • IGF-1 level (to screen for growth hormone excess)
  • Morning cortisol with ACTH after dexamethasone suppression
  • LH, FSH, and sex hormones if clinically indicated

Imaging Protocol

• Obtain dedicated pituitary MRI with gadolinium contrast using thin-section (3mm or less) coronal and sagittal sequences to better characterize this 3mm lesion, as standard brain MRI may have missed important details. 2, 1

• Visual field examination is NOT required for this 3mm microadenoma as it is too small to cause chiasmal compression (visual field testing is only indicated when lesions abut or compress the optic nerves/chiasm, which occurs with larger macroadenomas). 1

Follow-Up Strategy for Microincidentalomas

Since this is a microincidentaloma (<1cm), the surveillance approach differs significantly from macroadenomas:

• Repeat pituitary MRI at 1 year (not 6 months, as that interval is reserved for macroadenomas ≥1cm). 1

• If the lesion remains stable in size at 1 year, progressively lengthen imaging intervals (e.g., 2 years, then 3 years). 1

• Repeat endocrine testing at 1 year to ensure no development of hormonal hypersecretion. 1

• Clinical assessment every 6-12 months looking specifically for: 1

  • Headaches (new or worsening pattern)
  • Visual changes (though unlikely with 3mm lesion)
  • Signs of hormone excess: galactorrhea, acromegalic features, cushingoid features, hyperthyroid symptoms
  • Signs of hypopituitarism: fatigue, weight changes, cold intolerance, sexual dysfunction

When Surgery Would Be Indicated

This patient does NOT meet surgical criteria, but referral to neurosurgery would be warranted if: 1

• Visual field deficits develop
• Documented tumor growth on serial imaging
• Confirmed hormone hypersecretion (except prolactinoma, which is treated medically with dopamine agonists)
• Pituitary apoplexy with visual disturbance
• Lesion grows to abut the optic chiasm

Critical Pitfalls to Avoid

• Do not dismiss the low-normal cortisol of 6.4 μg/dL – this requires dynamic testing with dexamethasone suppression, as paradoxical non-suppression would indicate autonomous cortisol production despite "normal" baseline levels. 1

• Do not assume the mildly elevated TSH is simple primary hypothyroidism without excluding a TSH-secreting adenoma (check free T3, alpha-subunit if TSH remains elevated with normal/high free T4). 1

• Do not over-image – microincidentalomas require less frequent surveillance than macroadenomas, and excessive imaging increases cost without improving outcomes. 1

• Do not refer for surgery based on size alone – a 3mm lesion is far below any surgical threshold and most microincidentalomas remain stable indefinitely. 3, 1