Terminal to Vellus (T:V) Hair Ratios (Pool Party Analogy)
Today, we’ll talk about a slightly more advanced topic of terminal to vellus (T:V) hair ratios. Terminal hairs are thick (big hairs) and vellus hairs are tiny (thin) hairs. The normal ratio is typically between 4:1 and 7:1 in the healthy scalp. A T:V ratio of less than 4:1 is quite typical of a diagnosis of androgenetic alopecia.
But can this ratio ever rise higher? ... above 7:1? It comes as a surprise to many physicians and patients that the ratio of terminal hairs to vellus hairs increases in biopsies from patients with chronic telogen effluvium (CTE). It can rise above 8:1 in biopsies from CTE.
It’s difficult for many to understand this concept ... even the physicians I teach! The analogy I often use is a kids pool party on a hot summer day. Let’s say for the sake of argument that there are 15 children invited to the party and 3 adults supervising. That’s a children to adult ratio of 5:1, right? Let’s just call this the normal ratio of children to adults on the poolside deck. Now let’s imagine that it’s so incredibly hot outside that the only way anyone can deal with it is to periodically hop in the pool to cool off - that includes the children and the supervising adults. But not everyone wants to swim because there’s alot of fun and games on the deck as well. So kids may enter the pool at random times to cool off and the adults do too! There are periodically so many people entering and exiting the pool.
Now when we look at the deck at any given time we may see 12 children on the deck and 2 adults (a ratio of 6:1) or perhaps when it’s super hot we might even see 9 children on deck and 1 adult (a ratio of 9:1). One can easily see that the weather change has triggered an increase in the children to adult ratio on the deck (from 5:1 to 9:1). This is precisely what happens in CTE! The “trigger” of shedding leads to both terminal hairs and vellus hairs from leaving the scalp and an increase in the T:V ratio. Few people realize the power of measuring the T:V ratio. But now you do! A T: V ratio less than 4:1 is typical of androgenetic alopecia and above 8:1 is typical of pure CTE.
Dr. Jeff Donovan is a Canadian and US board certified dermatologist specializing exclusively in hair loss. To schedule a consultation, please call the Whistler office at 604.283.1887