T3-release from autonomously functioning thyroid nodules in vitro

 

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Summary

Toxic thyroid nodules have been shown to be of clonal origin. In a portion of them, point mutations affecting either the gene of the TSH receptor (TSHr) or the α-subunit of stimulating G-protein, consecutively leading to enhanced cAMP levels, which may enhance growth or functional activity of the thyrocyte or both, were recently found. To complement these studies, we evaluated hormone response (i.e. T3 release) in vitro from tissues derived from toxic thyroid nodules as compared directly to the surrounding paranodular tissues as well as tissues derived from euthyroid goiter and from patients with Graves' disease. Experiments were conducted in the presence and absence of bTSH or Graves' immunoglobulines. Tissues obtained during surgery were incubated over 5 h, followed by equilibrium dialysis for 24 h, and determination of free T3 in an aliquot by RIA. Basal T3 release in nodular tissues (n = 10) was significantly higher (median: 7.3 ng/1) compared to paranodular tissues (3.2 ng/1; P < 0.01), tissues derived from euthyroid goiter (1.3 ng/1; n = 12; P < 0.001) and thyroid tissues derived from patients with Graves' disease (2.5 ng/1; n = 6; P < 0.001). Upon stimulation with bTSH (1 IU/1), median T3 concentrations markedly increased to 11.5 ng/1 (P < 0.05), 7.3 ng/1 (P < 0.05), 4.2 ng/1 (P < 0.01) and 3.2 ng/1 (P = N.S.), respectively. Stimulation over basal values was 1.6-fold in nodular tissues, 2.3-fold in paranodular tissues, 3.2-fold in euthyroid goiter and 1.3-fold in Graves' disease. In toxic thyroid nodules basal hormonereleasing activities were stimulated by fifteen out of twenty (75%) Graves' sera tested. For comparison, stimulation in other tissues occurred in 45% (paranodular), 80% (euthyroid goiter) and 35% (Graves' disease), respectively.

In conclusion, tissue derived from toxic thyroid nodules exhibits enhanced basal hormone release as compared to both, the surrounding paranodular tissues and tissues from euthyroid goiter in vitro, which may reflect constitutional activation of TSHr, α-subunit of stimulating G-protein or other so far unknown intermediate by point mutations affecting the respective genes. Hyperactivities in toxic thyroid nodules may be even further enhanced by external stimulators such as TSH or TSH receptor antibodies. The first stimulator may have clinical relevance in patients with toxic thyroid nodules and not yet suppressed TSH; the latter could play a role in the rare Marine Lenhart syndrome.