Endocrine Abstracts

Background: Thyroglobulin (Tg) is a protein exclusively synthesized by follicular thyroid cells. Serum Tg levels depend on thyroid volume, serum TSH levels, gender, age, smoking, and iodine intake. Consequently, serum Tg reference range may exhibit variations among different populations. It is crucial to establish population-specific reference ranges, especially when interpreting serum Tg values in the follow-up of patients with differentiated thyroid cancer who have undergone lobectomy or total thyroidectomy without radioiodine ablation.

Objectives: The aim of our study was to assess Tg levels in euthyroid individuals without autoimmunity in an iodine sufficient area, and to identify factors affecting Tg in our population.

Methods: Participants were attendants of our outpatient endocrinology clinic. Screening included a complete medical history and physical examination, thyroid function tests (TSH, T3, T4, Tg, thyroid autoantibodies), urinary iodine excretion assessment, and thyroid ultrasound with estimation of the total thyroid volume.

Results: We selected 115 euthyroid and iodine sufficient individuals (65 women and 50 men), without thyroid autoimmunity, with a mean age of 36.93 (range 17-72) years. None of them had abnormal sonographic findings, and none had known thyroid disease or was on medication interfering with thyroid function. Tg levels were non-normally distributed in our population. The mean Tg level was 14.55 ng/mL (range: 0.39-56.74 ng/mL) and the median value was 12.38 ng/mL (interquartile range: 10.82 ng/mL), with 95% of the data falling in the interval 1.8-31.5 ng/mL. Tg levels were higher in smokers vs non-smokers (Median / IQR: 17.73 / 12.01 ng/mL vs 11.85 / 9.41 ng/mL, P=0.003). Tg was positively correlated with T3 (r=0.250, P=0.038) and T4 (r=0.235, P=0.012) levels. In multiple linear regression, the best model for Tg prediction had three regressors (smoking, T4, and TSH) and explained 11.7% of Tg variability. Non-linear models were able to explain >90% of Tg variance, but most of the explained variance was due to random effects.

Conclusion: We propose a reference range of 1.8-31.5 ng/mL for Tg in a euthyroid, iodine sufficient Greek population. Tg is higher in smokers, and is weakly correlated with T3 and T4 levels, but not with thyroid volume. Tg levels in our cohort seem to depend on unknown factors, other than those traditionally described in the literature.