Thyroid hormone concentrations and inhibition of thyroid stimulating hormone secretion in euthyroid subjects with autonomous thyroid nodules

Abstract. Sixty-four euthyroid patients with autonomous thyroid nodules and normal thyroxine (T₄) concentrations and tri-iodothyronine resin uptake have been studied. The serum tri-iodothyronine (T₃) concentration of the patients was 2.24 (±0.67) ng/ml, significantly higher than in a group of fifty-seven euthyroid control subjects (1.58 ± 0.30 ng/ml). When no extranodular tissue was visible on the basal thyroid scan, the T₃ was 2.31 (±0.63) ng/ml, significantly higher than in patients with some extranodular uptake on the basal scan (1.91 ± 0.42 ng/ml). There was no significant difference in the serum T₄ concentrations of the patients (7.37 ± 2.10 μg/100 ml) compared to the control group (6.88 ± 1.89 μg/100 ml). The T₄ concentrations were not correlated with total or partial inhibition of the extranodular tissue. The thyroid hormone concentrations were not directly correlated to the size of the nodule assessed by scan imaging. The thyroid stimulating hormone (TSH) concentration of the patients (1.52 ± 0.38 μU/ml) was significantly lower than in normals (2.49 ± 0.96 μU/ml). No significant difference was found in the TSH concentrations of patients with partial or total inhibition of extranodular tissue irrespective of the T₃ concentration. A thyrotrophin releasing hormone stimulation test in twelve patients did not increase the serum TSH, irrespectively of the T₃ concentration.
These data show the high frequency of elevated serum T₃ concentrations despite normal serum T₄ concentration in euthyroid patients with autonomous thyroid nodules. They confirm that inhibition of TSH secretion can occur when thyroid hormone concentrations are in the normal range.