What is the likely cause of widespread hair loss on my body over many years, including loss of hair on my toes, feet, lower legs, thighs, lower back, underarms, and scalp?

Widespread Body Hair Loss Over Many Years

The most likely cause of your widespread, gradual hair loss affecting multiple body sites (toes, feet, legs, thighs, lower back, underarms, and scalp) over many decades is androgenetic alopecia (pattern hair loss), potentially combined with age-related physiologic hair thinning, though peripheral vascular disease should also be considered given the prominent lower extremity involvement. 1, 2

Primary Diagnostic Considerations

Androgenetic Alopecia (Most Likely)

• Androgenetic alopecia is the most common form of hair loss, typically genetic, and relates to sensitivity to dihydrotestosterone (DHT) in genetically predisposed follicles. 3, 1
• This condition causes progressive, patterned hair thinning that can affect both scalp and body hair over many years, consistent with your decades-long timeline. 2, 4
• The gradual nature and multiple body sites affected align with this diagnosis rather than autoimmune causes. 1

Age-Related Physiologic Changes

• Normal aging causes progressive reduction in hair density and diameter across multiple body sites, which would explain the gradual multi-decade progression you describe. 2
• This process affects body hair as well as scalp hair and accelerates after middle age. 1

Peripheral Vascular Disease (Important to Exclude)

• Hair loss on the lower extremities (toes, feet, lower legs) is a classic sign of peripheral arterial disease and chronic ischemia. 5
• The prominent involvement of your lower extremities warrants vascular assessment, especially if you have cardiovascular risk factors. 5
• Look for other signs: cool extremities, diminished pulses, skin changes, or claudication symptoms. 5

Less Likely but Important Differential Diagnoses

Alopecia Areata (Less Likely Given Your Pattern)

• Alopecia areata is an autoimmune condition causing patchy, non-scarring hair loss, but typically presents with well-demarcated patches rather than diffuse gradual loss over decades. 6, 3
• About 20% have family history, and 34-50% recover within one year, which doesn't match your chronic progressive pattern. 6, 3
• This diagnosis would be unlikely given the absence of discrete patches and the very gradual progression. 6

Endocrine Disorders

• Hypothyroidism, hypopituitarism, and other endocrine disorders can cause diffuse hair loss affecting multiple body sites. 7
• These conditions typically present with additional systemic symptoms beyond hair loss alone. 7
• Thyroid disease should be excluded with TSH testing given its treatability. 3, 7

Nutritional Deficiencies

• Vitamin D deficiency shows strong association with hair loss, with 70% of alopecia areata patients having levels <20 ng/mL. 3, 8
• Iron deficiency (low ferritin) causes chronic diffuse telogen hair loss and is the most common nutritional deficiency worldwide. 3
• Zinc deficiency impairs hair follicle function and tends to be lower in patients with hair loss. 3, 8

Recommended Diagnostic Approach

Essential Clinical Evaluation

• Examine for vascular insufficiency signs: check pedal pulses, capillary refill, skin temperature, and ankle-brachial index if lower extremity pulses are diminished. 5
• Assess scalp for pattern of hair loss: diffuse thinning at crown with frontal hairline preservation suggests androgenetic alopecia. 3
• Look for exclamation mark hairs (short broken hairs) which would indicate alopecia areata, though this seems unlikely given your presentation. 3, 8

Targeted Laboratory Testing

• Check TSH to exclude thyroid disease, which is readily treatable and can cause widespread hair loss. 3, 7
• Measure serum ferritin, as iron deficiency is a common reversible cause of chronic hair loss. 3
• Test vitamin D levels (deficiency defined as <20 ng/mL), given the strong association with hair loss. 3, 8
• Consider checking serum zinc and folate if other tests are unrevealing. 3

When to Pursue Further Workup

• If vascular disease is suspected, obtain vascular studies and cardiology consultation. 5
• If endocrine disorder is suspected based on systemic symptoms, expand hormonal evaluation. 7
• Dermoscopy or scalp biopsy is rarely needed for your presentation but would be indicated if diagnosis remains uncertain. 3, 4

Common Pitfalls to Avoid

• Don't overlook peripheral vascular disease as a cause of lower extremity hair loss—this can indicate significant cardiovascular risk requiring intervention. 5
• Avoid ordering excessive laboratory tests when the clinical picture clearly suggests androgenetic alopecia or age-related changes. 3
• Don't assume all hair loss is benign—systemic diseases including endocrine disorders and vascular disease can present primarily with hair loss. 5, 7
• Consider the psychological impact, as hair loss can cause significant distress and affect quality of life, warranting empathetic discussion of realistic expectations. 1, 2