Toenails often become thicker with age or thicken due to trauma or injury, according to the American College of Foot and Ankle Surgeons. Nail fungus, psoriasis and hypothyroidism can all thicken toenails.
A fungal infection that affects one or more nails is called onychomycosis, notes Healthline. Risk factors for developing onychomycosis include having diabetes, wearing artificial nails, having a nail injury and wearing closed-toe shoes. An infection can cause nail yellowing, nail loss and scaling under the nail.
Psoriasis is a common skin condition that can affect the nails and occurs when the body produces new skin cells within days rather than weeks, explains Healthline. The condition results in red, dry and itchy skin. As of 2015, research suggests that it is an autoimmune disorder influenced by genetics. Risk factors for the disease include obesity, smoking and a family history of the condition.
Hypothyroidism occurs when the thyroid gland produces an inadequate amount of the thyroid hormone, according to MedlinePlus. Thyroiditis, viral infections, pregnancy, certain medications, radiation treatments and radioactive iodine treatment can cause this condition. Early symptoms of hypothyroidism include constipation, fatigue, weakness and weight gain. If left untreated, hypothyroidism can result in decreased taste, slow speech and thickening of the skin.