Dips in fingernails are almost always caused by a disruption to the nail matrix, the tissue just beneath your cuticle where new nail cells form. The specific pattern of the dip, whether it’s a horizontal groove, a tiny pit, or a scooped-out curve, points to different underlying causes ranging from a passing illness to a nutritional deficiency. Understanding what type of dip you’re looking at is the first step to figuring out what triggered it.
Three Main Types of Nail Dips
Not all nail dips are the same, and the differences matter. The three most common patterns each have distinct causes:
- Horizontal grooves (Beau’s lines): Depressions that run side to side across the nail, like a ditch carved into the surface. These form when nail growth temporarily slows or stops due to illness, stress, or injury.
- Small pits: Tiny, dot-like depressions scattered across the nail surface. These are linked to skin conditions like psoriasis and alopecia areata.
- Spoon-shaped dips (koilonychia): The nail thins, curves inward, and develops a scooped appearance. This is most commonly a sign of iron deficiency.
Horizontal Grooves and What They Reveal
Beau’s lines are the most common type of nail dip people notice. They appear as one or more horizontal indentations running across the nail plate. Each groove represents a moment when something disrupted the nail matrix enough to slow cell production, creating a visible “pause mark” in the nail.
A wide range of triggers can cause them. Acute illnesses with high fevers, including pneumonia, measles, mumps, and strep infections, are classic causes. COVID-19 has also been linked to Beau’s lines. More serious events like heart attacks can produce them too. Chronic conditions that reduce blood flow to the fingers, such as diabetes, hypothyroidism, peripheral artery disease, and Raynaud’s phenomenon, are another category of triggers. Severe zinc deficiency or inadequate protein intake can also be responsible.
What surprises many people is that emotional stress alone can cause these grooves. Major life disruptions like a death in the family, divorce, or job loss can affect nail growth enough to leave a visible mark. Anxiety disorders are another possible cause.
Because fingernails grow at an average rate of about 3.5 millimeters per month, you can roughly estimate when the disruption occurred by measuring the distance from the groove to the cuticle. A groove sitting 7 millimeters from the base of your nail, for example, likely formed about two months ago. If the same groove appears on multiple nails at the same position, that strongly suggests a systemic event (like a fever or illness) rather than a local injury to one finger.
Nail Pitting and Skin Conditions
Nail pitting looks different from horizontal grooves. Instead of a single line, you’ll see scattered small depressions across the nail surface, almost like someone pressed a pin tip into it repeatedly. These pits form when clusters of abnormal cells in the nail matrix produce defective nail material that flakes away easily, leaving tiny craters behind.
Psoriasis is the condition most strongly associated with nail pitting. The same inflammatory process that causes scaly skin patches disrupts normal nail cell production in the matrix. If you already have psoriasis on your skin, nail pitting is a common companion. But nail pitting can also be the first sign of psoriasis, appearing before any skin changes show up.
Alopecia areata, an autoimmune condition that causes hair loss, is the other major cause. Nail changes appear in roughly 22% of people with alopecia areata on average, though rates climb higher in more severe forms. The pitting pattern differs subtly from psoriasis: pits tend to be shallower and arranged in a more regular, grid-like pattern, sometimes creating an overall wavy texture rather than distinct craters. The inflammatory process disrupts how nail cells mature but doesn’t cause permanent scarring of the matrix, so the nail can recover if the underlying condition is treated.
Spoon-Shaped Nails and Iron Deficiency
Koilonychia has a distinctive look. Rather than a groove or pit, the entire nail becomes thin, develops raised ridges, and curves inward so it could hold a drop of water like a tiny spoon. In many people, this is a direct sign of iron deficiency anemia.
Iron plays a role in producing the tough protein (keratin) that makes up your nail plate. When iron stores drop low enough, the nail structure weakens and loses its normal convex shape. Vitamin B deficiencies can contribute as well. If iron deficiency is the cause, correcting it through dietary changes or supplements typically allows normal nail shape to return as new nail grows in, though it takes several months to see the full result since fingernails need roughly four to six months to grow out completely.
Spoon nails can occasionally result from other factors, including repeated exposure to petroleum-based solvents or certain occupational chemicals, but iron deficiency remains the most common explanation by a wide margin.
Damage From Picking or Rubbing
Sometimes the cause is purely mechanical. Habit-tic deformity is a condition where someone repeatedly picks at, pushes back, or rubs the skin fold at the base of the nail, often unconsciously. This damages the nail matrix and produces a characteristic “washboard” pattern: a central groove running down the length of the nail with parallel horizontal ridges layered on top. The cuticle may be missing or the half-moon at the nail base may look enlarged.
This pattern most commonly affects the thumbnails and is often asymmetrical, appearing worse on one hand. Because it’s caused by an ongoing habit rather than a one-time event, the depressions keep forming until the behavior stops. Many people don’t realize they’re doing it, which makes it easy to overlook as a cause.
Rough, Sandpaper-Like Nails
A less common but more dramatic pattern is trachyonychia, sometimes called “sandpaper nails.” The nails become thin, brittle, and covered in excessive longitudinal ridging, giving them a rough, sandpaper-like texture. This can affect just a few nails or all twenty. It’s most often associated with the same conditions that cause pitting (psoriasis, alopecia areata, and eczema) but represents a more widespread disruption to the nail matrix.
How to Read Your Nails
A single Beau’s line on one nail after you jammed your finger in a door is straightforward and nothing to worry about. The nail will grow out normally. But certain patterns carry more weight. Horizontal grooves appearing on multiple nails at once suggest your whole body went through something, whether that was a high fever, a period of severe stress, or a nutritional shortfall. Persistent pitting, especially with any skin or hair changes, points toward an autoimmune or inflammatory condition worth investigating. Spoon-shaped nails warrant checking your iron levels with a simple blood test.
The 3.5 millimeter per month growth rate gives you a useful timeline. A complete fingernail takes roughly four to six months to grow from base to tip, so whatever caused a dip near the middle of your nail happened two to three months ago. If new nail growing in from the base looks healthy, the disruption was likely temporary and has already resolved. If new growth continues to look abnormal, the underlying cause is probably still active.