The effect of T4 on intrafollicular clock activity is likely to have a functional impact on HF biology, since qualitatively it appears to induce circadian rhythmicity of intrafollicular cyclin D1 expression (non-significant) and BMAL1 and PER1 are expressed in the bulge, the niche for HF epithelial stem cells. The gene discussed is PER1; the disease is hydrops fetalis.