A toenail that has turned dark and thick is most often caused by a fungal infection, but it can also result from trauma, skin conditions like psoriasis, or in rare cases something more serious. The combination of color change and thickening together is a strong clue to the cause, and understanding what’s behind it helps you figure out whether it needs treatment or will resolve on its own.
Fungal Infection: The Most Common Cause
Fungal nail infections account for the majority of thick, discolored toenails. The fungi responsible feed on keratin, the protein your nails are made of. They typically enter through the space where the nail meets the skin at the tip of your toe, then spread backward toward the base. As the infection takes hold, your nail bed produces extra layers of keratin in an attempt to shed the invader. This buildup, called subungual hyperkeratosis, is what makes the nail visibly thicker. The color shifts to yellow, brown, or dark because of the fungal debris trapped between the nail and the nail bed.
In chronic infections, the buildup becomes dense and compact, and the nail may start to separate from the bed entirely. The nail can become crumbly at the edges and develop a foul smell. Fungal nail infections are extremely common in older adults. Roughly 20% of people aged 60 and older have one, and the rate climbs to over 50% in those 70 and older. Risk factors include diabetes, poor circulation, a history of athlete’s foot, and spending time in warm, damp environments like public showers or pool decks.
Trauma and Repetitive Pressure
A single injury, like dropping something heavy on your toe or stubbing it hard, can cause blood to pool beneath the nail. This trapped blood (a subungual hematoma) starts out red or purple, then gradually turns dark brown or black. The nail may also thicken as it grows out. A bruised toenail can take six to nine months to fully grow out and return to normal, so the dark color lingers much longer than you might expect.
Repetitive low-grade trauma is just as common and easier to miss. Running, hiking, or wearing shoes that are too tight creates constant pressure on the toenails, especially the big toe. Over time, this repeated impact disrupts the connection between the nail and the tissue that produces it. New nail layers pile up rather than growing out smoothly, resulting in a thicker, yellowed nail plate. Runners, hikers, and people who spend long hours on their feet in narrow shoes are particularly prone to this. If the damage is ongoing, the nail can stay thick and discolored indefinitely.
Nail Psoriasis
Psoriasis can affect toenails in ways that look a lot like a fungal infection, with thickening, discoloration, and crumbling. But there are a few visual markers that set it apart. Psoriasis often causes tiny pits across the nail surface, small depressions that look like someone pressed a tack into the nail. It also produces reddish or dark brown splotches underneath the nail called “oil spots.” These oil spots do not appear in fungal infections, which makes them a reliable way to tell the two apart. If you already have psoriasis on your skin or scalp, there’s a higher chance your nail changes are related to the same condition rather than a fungus.
Circulation Problems
Poor blood flow to your feet can change how your toenails grow. Peripheral artery disease, where narrowed arteries reduce circulation to the legs and feet, can cause toenails to thicken, grow slowly, or stop growing altogether. You might also notice that leg hair has thinned or disappeared, your feet feel cold, or you get cramping in your calves when walking. Diabetes can produce similar nail changes through a combination of reduced circulation and nerve damage. If your thick, dark toenail comes alongside any of these other symptoms, the underlying cause may be vascular rather than local.
When Dark Color Is a Red Flag
A dark streak running lengthwise through the nail deserves prompt attention. While many dark streaks are harmless, a new or changing streak can be a sign of subungual melanoma, a rare but serious form of skin cancer that develops under the nail. The risk is higher if you’re between 50 and 70, if the streak is wider than 3 millimeters with an irregular border, or if it appears on your big toe or thumb. One key warning sign is pigment that extends beyond the nail itself and bleeds into the surrounding skin, known as the Hutchinson sign. People of African, Asian, or Native American descent have a higher incidence of this type of melanoma.
A dark streak that’s growing, changing shape, or spreading beyond the nail edge should be evaluated by a dermatologist. Early detection makes a significant difference in outcomes.
How Fungal Infections Are Diagnosed
Your doctor or podiatrist won’t diagnose a fungal nail infection by appearance alone, because psoriasis, trauma, and other conditions can look nearly identical. The standard test involves clipping a piece of the affected nail and scraping some of the debris from underneath it. That sample is examined under a microscope after being treated with a solution that dissolves everything except fungal cells, making them easy to spot. A fungal culture may also be sent to a lab to identify the exact species, which can take a few weeks to come back.
What Treatment Looks Like
If the cause is a fungal infection, treatment depends on severity. Mild cases sometimes respond to topical antifungal solutions applied directly to the nail, though these have limited ability to penetrate a thick nail plate. For moderate to severe infections, oral antifungal medication taken daily for about 12 weeks is the standard approach. Even after the medication course ends, visible improvement is slow because you’re waiting for an entirely new, healthy nail to grow in. That process takes anywhere from 9 to 18 months for a toenail, so patience matters.
If the problem is repetitive trauma, switching to properly fitted shoes with a roomy toe box is the first step. The damaged nail will need to grow out completely before you see a normal one replace it. For psoriasis-related changes, treatment typically targets the underlying immune condition rather than the nail itself. And for a simple bruise under the nail, no treatment is needed. The discoloration grows out on its own over several months.
Signs That Need Professional Evaluation
Certain nail changes warrant a visit to a dermatologist or podiatrist sooner rather than later:
- A new or changing dark streak running the length of the nail
- Pigment spreading into the skin around the nail
- A greenish-black color, which can indicate a bacterial infection
- Redness and swelling around the nail, suggesting active infection
- Nails that have become extremely thick and curved (sometimes called ram’s horn nails), which require professional trimming and may signal an underlying condition
- Nail changes alongside symptoms of poor circulation, such as cold feet, leg cramping, or slow-healing wounds
A thick, dark toenail that’s been stable for months and clearly started after an injury is less concerning than one that appeared without explanation or is actively changing. When in doubt, a nail clipping test is quick and painless, and it gives you a definitive answer about whether a fungus is involved.