[Genesis of thyroid nodules. Physiological and pathological mechanisms, clinical implications].

Thyroid nodules are the most frequently encountered thyroid diseases since they are displayed by clinical examination and ultrasound thyroid study in 3-5% and 50% of any patients, respectively. Thyroid nodules are more frequent in females and their prevalence increases with age, iodine deficiency and exposure to radiation. Thyrocytes proliferation is under the control of two classic factor (TSH and iodine) and a dozen of newly identified growth factors/regulators. Indeed, the hyperplastic phase, its angiogenic counterpart, and the following expression of thyrocytes' differentiation are the result of a complex intercellular cooperativity. Moreover, the innate thyrocytes heterogeneity, regarding their tendency to proliferate and differentiate, determines a more complexified control. This complex mechanism which allows a fine tuning of the thyroid function is nevertheless responsible for the various targets that will be affected by manifold etiopathogenetic factors. Thyroid nodules genesis may thus be envisioned as an amplification, secondary to genetic or/and epigenetic mechanisms, of the thyroid gland heterogeneity. Experimental data and their conceptual outcomes should be kept in mind in the diagnostic, therapeutic or preventive care of thyroid nodular diseases.