What are the possible causes of selective weight gain in the thighs and buttocks with accompanying fatigue?

Selective Weight Gain in Thighs/Buttocks with Fatigue: Differential Diagnosis

The most likely diagnosis to consider is Acquired Partial Lipodystrophy (Barraquer-Simons syndrome), which characteristically presents with fat accumulation in the hips, buttocks, and legs while losing fat from the upper body, though other conditions including hypothyroidism, Cushing's syndrome, and familial lipomatosis patterns must be ruled out. 1

Primary Diagnostic Considerations

Acquired Partial Lipodystrophy (APL)

This condition is the most specific match for the described pattern. APL typically:

• Affects females 4 times more than males 1
• Begins in childhood or adolescence (though can occur later)
• Shows cranio-caudal fat loss (face, neck, shoulders, arms, trunk lose fat)
• Results in compensatory fat accumulation in hips, buttocks, and legs 1
• Associated with autoimmune diseases in ~20% of cases, particularly membranoproliferative glomerulonephritis 1
• Causes fatigue through metabolic derangements and insulin resistance 1

Key diagnostic steps:

• Check serum complement 3 (C3) levels - typically low in APL 1
• Screen for C3 nephritic factor
• Evaluate for autoimmune diseases
• Physical examination should document the cranio-caudal pattern of fat distribution

Secondary Causes to Exclude

Hypothyroidism - Critical to rule out first:

• Causes generalized weight gain, fatigue, and lethargy 2
• Check TSH, free T4, T3 uptake, and thyroid-binding globulin 2
• 20-25% of patients with unexplained weight gain and fatigue may have undiagnosed hypothyroidism 2

Cushing's Syndrome:

• Can mimic lipodystrophy with truncal obesity 1
• Look for: thin atrophic skin, purple striae, buffalo hump, supraclavicular fat pads, facial plethora 3, 4
• Check 24-hour urinary free cortisol or late-night salivary cortisol
• Note: Familial partial lipodystrophy can present with a "Cushingoid appearance" but has opposite fat distribution (fat loss in limbs/buttocks, not gain) 1

Polycystic Ovarian Syndrome (PCOS):

• Associated with insulin resistance and lipodystrophy syndromes 1
• Check for hirsutism, acanthosis nigricans, menstrual irregularities 3
• Measure serum estradiol and testosterone levels 5

Clinical Examination Priorities

Physical findings to document:

1. Fat distribution pattern - precisely map where fat is increased vs. decreased
2. Acanthosis nigricans (neck, axillae, groin) - indicates insulin resistance 1
3. Muscular hypertrophy - common in lipodystrophy syndromes 1
4. Skin changes - striae, thinning (Cushing's), or normal
5. Facial features - fat accumulation or loss pattern

Laboratory Workup Algorithm

First-tier tests:

• TSH, free T4, T3 uptake, TBG
• Fasting glucose, HbA1c
• Lipid panel (triglycerides often severely elevated in lipodystrophy) 1
• Comprehensive metabolic panel
• Serum complement C3 level 1

Second-tier if APL suspected:

• C3 nephritic factor
• Autoimmune panel (ANA, anti-dsDNA)
• Renal function and urinalysis (screen for glomerulonephritis)
• Consider dual-energy x-ray absorptiometry or MRI to document fat distribution 1

If Cushing's suspected:

• 24-hour urinary free cortisol
• Late-night salivary cortisol
• Low-dose dexamethasone suppression test

Critical Pitfalls to Avoid

1. Do not dismiss as simple obesity - The selective lower body distribution with fatigue is pathognomonic for specific syndromes 1, 5

2. Do not overlook thyroid dysfunction - This is the most common treatable cause and should be excluded first 2

3. Recognize that metabolic complications are uncommon in APL unlike other lipodystrophy forms, though insulin resistance can still occur 1

4. Screen for serious complications - APL patients are at risk for membranoproliferative glomerulonephritis which can progress to renal failure 1

5. Differentiate from familial patterns - Familial partial lipodystrophy causes fat loss (not gain) in limbs and buttocks, opposite of what's described 1

Metabolic Consequences Requiring Monitoring

Even if metabolic complications are less common in APL than other lipodystrophy forms, monitor for:

• Diabetes mellitus (insulin resistance from ectopic fat storage) 1
• Hypertriglyceridemia (can be severe, risk of pancreatitis) 1
• Nonalcoholic fatty liver disease 1
• Cardiovascular disease risk 1

The fatigue in this presentation likely stems from the underlying metabolic derangements, potential autoimmune disease, or undiagnosed hypothyroidism rather than the fat distribution itself 1, 2.