Thirty-nine thyroid nodules, removed because of recent growth, were analyzed morphologically by serial histological sections for the classical histomorphological hallmarks of follicular cell replication and for immunohistochemically demonstrable overexpression of the growth-associated ras-gene product p21ras. Clonal analysis was performed using the highly informative probe M27 beta that detects polymorphisms on the locus DXS255 of the X-chromosome. Twenty-four nodules were of clonal and 15 nodules were of poly-clonal origin. Only 3 out of the 24 clonal nodules were histomorphologically uniform. In all others, the structural hallmarks of active growth and the P21ras growth-marker expression were remarkably heterogeneous throughout the tumors. There were no histomorphological characteristics distinguishing these clonal tumors from polyclonal nodules. Even if a clonal thyroid tumor may be originally homogeneous in respect to the parameters studied here, mechanisms must exist that create wide heterogeneity of growth and of morphogenetic potential among the individual follicular cells during further expansion of the nodule. Thus, clonal nodules are much more common in nodular goiters than hitherto assumed on grounds of the classical morphological criteria. The diagnosis of a true monoclonal nodule can no longer rely on morphological and functional criteria alone but requires molecular or cytogenetic analysis of clonality.

Although somatic mutations have been identified in a subset of thyroid nodules, the pathogenesis of nodules in multinodular goiters remains unclear. Clonal analysis indicates whether a nodule arises from the polyclonal proliferation of a group of cells or forms a clone from a genetically altered cell. Individual thyroid nodules have been shown to be of polyclonal or monoclonal origin. In this study we examined the clonality of several different nodules in patients with multinodular goiters. Clonality was established using the X-chromosomal probe M27 beta, which detects a multiallelic polymorphism at the locus DXS255 in 90% of females. Twenty-five nodules from 9 multinodular goiters were analyzed; 9 nodules were polyclonal, and 16 were monoclonal. Three goiters contained only polyclonal nodules, whereas 3 contained only monoclonal nodules. Polyclonal and monoclonal nodules coexisted in 3 goiters. In 2 goiters, the monoclonal nodules were shown to derive from different progenitor cells. We conclude that polyclonal and monoclonal nodules may coexist in multinodular goiters and that monoclonal nodules can originate from different cells. The coexistence of polyclonal and monoclonal nodules suggests that different pathogenic mechanisms occur simultaneously or that monoclonal nodules emerge secondarily from a polyclonal population due to a growth advantage from a genetically altered cell.

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Iodine may have an inhibitory and, in some circumstances, a stimulatory effect on thyroid follicular cell growth. Exogenous iodine deficiency causes the growth of endemic goitres and it has been claimed that low intrathyroidal iodine stores stimulate growth. On the other hand, the role of iodine, if any, in regulating the growth of human nodular goitres exposed to an ample supply of iodine has not been studied systematically. Very few data on intrathyroidal iodine concentration in this type of goitre are available. In the present work we have investigated total iodine content in 24 samples from 11 clinically and histomorphologically well-defined fast and autonomously growing human nodular goitres from a non-endemic area. Iodine was fractionated into thyroglobulin-iodine and non-thyroglobulin-iodine. The regional distribution of intrathyroidal iodo-compounds was also assessed in three goitres. Total iodine concentration, as well as its sub-fractions, i.e. thyroglobulin-iodine and non-thyroglobulin-iodine, were significantly lower than in normal thyroids. Furthermore, there was large inter- and intraindividual heterogeneity of all iodo-compounds as well as of thyroglobulin. Total iodine concentration varied by a factor of almost 40 between different goitre samples and by a factor of 20 between samples taken from the same goitre. Total non-thyroglobulin-iodine, the only fraction comprising possible cell growth-regulating iodo-compounds, varied by a factor of> 60 between different goitres and by a factor of > 6 between different samples of the same goitre. The low iodine concentration in all our goitre samples did not differ from values reported in the literature for endemic iodine-deficient goitres. Since all goitres studied here were actively growing while exposed to an ample supply of iodine, iodine shortage cannot be a primary and causal factor for the growth of this type of sporadic goitre. Rather, the low concentration and the large inter- and intraindividual heterogeneity of all iodo-compounds appear to be secondary incidental events well explained by the recently developed concept of autonomous thyroid growth.