<p>Microtubules are long filaments that control cellular structure and influence intracellular transport. The female hormone estrogen has been implicated in alterations to the microtubule network for a variety of cell types. However, the effects of estrogen on individual microtubules are unknown. In this work we systematically investigated a mechanism by which estrogen could alter the length dynamics of individual microtubules. Using multi-line cell assays, cell-free experiments, and computational modeling, we found that estradiol acts to frustrate and pause microtubule growth in cells, independent of estrogen receptor pathways. Specifically, estradiol acts as a switch in which dynamic, growing microtubules were transformed into paused, non-growing microtubules. Estradiol pauses microtubule growth without inducing an increased incidence of catastrophe events, similar to the widely used microtubule poison colchicine. We conclude that estrogen’s ability to limit excessive microtubule proliferation could have important implications for therapeutic approaches in heart disease and breast cancer.</p>