Thyroid gland function is thought to be tightly regulated by thyrotropin (TSH). However, the function of each thyroid follicle is heterogeneous despite having the same blood supply of TSH and despite homogeneous thyrocyte expression of the TSH receptor (TSHR). The nature of this heterogeneity is not fully understood. Recent studies showed that thyroglobulin (TG) protein, stored in the follicular lumen, is a potent negative feedback regulator of follicular function. Thus, physiological concentrations of TG significantly suppress thyroidspecific gene expression in cultured thyrocytes and antagonizes the maximal TSH stimulation of thyroid-specific genes: thyroglobulin, thyroid peroxidase, sodium iodide symporter and TSH receptor. In vivo studies are consistent with these results. This regulation is mediated by TG suppression of thyroid-specific transcription factors: thyroid-transcription factors 1 and 2 as well as Pax-8. We propose a model of follicular activity wherein each follicle has its own cycle of thyroid hormone synthesis and secretion and wherein follicular heterogeneity reflects the asynchronous function of individual follicles. We provide evidence that this mechanism may be related to the phenotype of some thyroid diseases.