Endocrine Abstracts

Background: Central hypothyroidism can be difficult to diagnose as TSH values often reside within the normal range. The Endocrine Society clinical guideline for hormone replacement in hypopituitarism suggests levothyroxine is indicated for treatment of secondary hypothyroidism where fT4 levels decrease by ≥20%. To determine the reliability of this biochemical marker of secondary hypothyroidism we evaluated evolution of TFTs over time within a cohort of NFPA.

Methods: A retrospective review of TFTs was undertaken during 2000 to mid-2019 whilst using the Siemen’s Centaur assay to measure fT4 levels. Patients with NFPA were identified from the departmental database. Patients with a history of CNS/pituitary irradiation; more than one pituitary surgery; on levothyroxine or identified with primary or secondary hypothyroidism at their first assessment during the study period, were excluded from analysis. Demographic, adenoma characteristics, treatment and hormone data were collected from primary and secondary care records.

Results: A total of 67 patients were identified, 39 male and 28 female with a mean age 58.0 (range 29.0-89.7) years. 34 patients underwent surgery and 33 patients did not. Mean follow-up was 5.2 (range 0.25-15.1) years. The indication for surgery in the majority of patients was visual compromise from optic chiasm compression. A total of 22/67(32.8%) patients experienced a drop of ≥20% in their fT4 levels from baseline (either first thyroid function test at diagnosis or first post-operative result in those who underwent surgery). Of the 22 patients, 12 showed subsequent recovery in their fT4 levels, whereas 10 did not recover during follow-up. In those who showed recovery 7, 2 and 3 had no, one or two hormone deficits. Similarly in those who did not recover 6, 2 and 2 had no, two or three additional hormone axis deficits. 12/67 patients saw a drop in their fT4 to below the reference range (10-20pmol/l), of whom 7 also showed a ≥20% decrease in fT4 values. 5 showed recovery of their fT4 levels and 7 a temporary decrease.

Conclusions: Variations of ≥20% in fT4 values occur in around a third of operated and non-operated NFPA on long-term follow-up. Around 50% of these episodes resolve spontaneously. Current guidelines may therefore lead to over diagnosis of secondary hypothyroid in patients with pituitary adenoma. Surveillance of repeated fT4 levels and symptoms is indicated before a diagnosis of secondary hypothyroidism can be established.