Terminal-to-Vellus Ratio: The Number That Defines Your Progress

A normal scalp has a terminal-to-vellus ratio above 4:1. Below 2:1, the AGA diagnosis is confirmed regardless of the clinical appearance. Treatment response shifts this ratio measurably before clinical improvement is visible.

The terminal-to-vellus ratio is arguably the most quantitatively precise measure of AGA stage and the clearest marker of treatment response at the follicle level. It captures the core biology of AGA — progressive follicle miniaturisation from terminal to vellus — in a single numerical ratio that can be tracked over time.

Measuring the T/V ratio

The T/V ratio can be assessed through two methods:

Horizontal scalp biopsy: The gold standard. A 4mm punch biopsy is processed horizontally at three levels (infundibulum, isthmus, bulb), providing a cross-sectional view of all follicular units in the sample. Terminal follicles are identified by hair shaft diameter above 60μm; vellus follicles by shaft diameter below 30μm. Miniaturised intermediate follicles (30–60μm) are counted separately. A pathologist experienced in horizontal section analysis provides the quantitative count. This is the method by which Whiting established the T/V ratio diagnostic criteria for AGA.

Trichoscopy: Non-invasive dermoscopy can estimate T/V ratio by quantifying the proportion of fine vellus-like hairs visible in the thinning zone. While less precise than biopsy (dermoscopy cannot access the deeper follicle calibre directly), experienced trichoscopic assessment provides a clinically useful estimate and can track change over treatment cycles.

Staging and treatment monitoring

T/V ratio provides a disease-stage metric independent of clinical pattern. A man with diffuse AGA and T/V ratio 1.8:1 is significantly more miniaturised than a man with classic temporal recession but a T/V ratio of 3.5:1 — a distinction not captured by Norwood staging.

Treatment response on biopsy: finasteride studies have shown measurable increases in T/V ratio and hair shaft diameter at 12–24 months of treatment, before clinically visible density improvements are clearly apparent. The biopsy or trichoscopic T/V shift provides earlier evidence of treatment response than photographic density changes for borderline responders.