Endocrine Abstracts

Background: Diagnosing secondary hypothyroidism can be challenging, particularly for patients with low-normal free T₄ (fT₄). Thyroid hormone replacement (THR) in these cases is guided by the normal population fT₄ range. The aim of this study was to compare thyroid function tests in thyroid cancer patients on THR and patients with pituitary diseases with or without central hypothyroidism.

Methods: We retrospectively collected data from low-risk thyroid cancer patients, post-total thyroidectomy with TSH 0.2–2.0 miu/l and patients with hypopituitarism, with or without TSH deficiency, who were reviewed in Outpatient Clinics during March 2015-February 2016. Patients with primary hypothyroidism/hyperthyroidism and TSHomas were excluded. The fT4 reference range for our laboratory is 10.0–20.0 pmol/l.

Results: 119 patients with low risk thyroid cancer (Group 1, age 55.5±16.1 years), 144 hypopituitary patients with TSH deficiency on THR (Group 2, age 60.9±15.1 years) and 79 hypopituitary patients not on THR (Group 3, age 56.1±13.7 years) were studied. Mean fT₄ was higher in Group 1 compared with Group 2 (19.1±3.1 vs 15.9±2.9 pmol/l, P<0.001) and Group 3 (19.1±3.1 vs 13.4±2.0 pmol/l, P<0.001). Additionally, patients in Group 2 had significantly higher mean fT₄ compared with Group 3 (15.9±2.9 vs 13.4±2.0 pmol/l, P<0.001). The distribution of fT₄ values in Group 2 was similar to the normal population fT₄ range. In contrast, a right-sided shift of fT₄ distribution was observed in Group 1 and a left-sided shift was noted in Group 3. Only 1.6% of patients in Group 1 had fT₄ values of <13 pmol/l, compared with 19.4% in Group 2 and 43% in Group 3.

Conclusion: Low risk thyroid cancer patients on THR demonstrate a different fT₄ distribution range compared with patients with pituitary diseases and normal population, while maintaining normal TSH levels. The higher fT₄ range in this thyroid cancer group may guide THR in pituitary patients to avoid under-diagnosis and under-treatment of central hypothyroidism.