Weight Loss Resistance in an Active Woman with Normal Thyroid Function
This patient's inability to lose weight despite regular exercise and healthy eating most likely reflects inadequate caloric deficit—she is consuming more calories than she realizes, as the vast majority of people significantly underestimate their intake and overestimate their activity level. 1
Primary Issue: Hidden Caloric Surplus
The central problem is almost certainly energy balance, not metabolic dysfunction:
The American Diabetes Association emphasizes that most people dramatically underestimate caloric intake and overestimate physical activity, making a true caloric deficit rare without objective documentation. 1 Her TSH of 2.87 mIU/L is well within normal range, effectively ruling out hypothyroidism as a cause. 2
Body weight is regulated by the central nervous system through powerful biological mechanisms that resist weight loss, particularly in those genetically predisposed to maintaining higher weight. 2, 1 Her 32-pound gain over 8 years (approximately 4 pounds/year) suggests gradual positive energy balance, not sudden metabolic collapse.
To achieve actual weight loss, she must create a documented deficit of 500-750 kcal/day below her true maintenance needs, which for most women means consuming 1,200-1,500 kcal/day. 1
Secondary Consideration: Low-Normal Ferritin
Her ferritin of 36 ng/mL warrants attention:
Iron deficiency (even without anemia) affects 24-47% of women at baseline and can worsen metabolic efficiency by impairing T4 to T3 conversion and shifting ATP production toward less efficient anaerobic pathways. 2 This could contribute marginally to her difficulty losing weight by reducing exercise efficiency and potentially lowering metabolic rate. 2
Iron deficiency impairs thyroid hormone metabolism through effects on Type 1 5'-deiodinase, a selenoenzyme required for hepatic T4 to T3 conversion. 2 While her TSH is normal, suboptimal iron stores may still affect peripheral thyroid hormone action.
Structured Diagnostic and Treatment Approach
Step 1: Document Actual Caloric Intake (Non-Negotiable)
Require detailed daily food logs with measured portions for at least 2 weeks—patients who regularly record food intake lose significantly more weight than those who don't. 1 This is the single most important intervention.
Use a food scale and measuring cups; estimate portion sizes are notoriously inaccurate. 1
Calculate her true maintenance calories: At 192 lbs (87 kg) with 4-5 exercise sessions weekly, her maintenance is approximately 2,000-2,200 kcal/day. Target 1,400-1,500 kcal/day for 1-1.5 lb/week loss. 1
Step 2: Address Iron Deficiency
Supplement with elemental iron 65-130 mg daily (ferrous sulfate 325 mg contains 65 mg elemental iron) to optimize ferritin above 50 ng/mL, which may improve metabolic efficiency and exercise tolerance. 2
Recheck ferritin in 8-12 weeks; iron deficiency can perpetuate metabolic inefficiency through multiple mechanisms. 2
Step 3: Optimize Exercise Strategy
Exercise alone produces minimal weight loss—it is crucial for maintaining weight loss once achieved, but cannot compensate for excess caloric intake. 1 Her 4-5 sessions weekly are excellent for health but won't overcome caloric surplus.
Recommend 200-300 minutes weekly of moderate-intensity activity plus resistance training 2-3 times weekly to preserve lean muscle mass during caloric restriction. 1 This prevents metabolic slowdown from muscle loss.
Step 4: Implement Behavioral Support
Standard dieting advice without intensive support produces poor long-term outcomes—she needs high-frequency counseling (weekly initially) focused on behavioral strategies, self-monitoring, and addressing specific barriers. 1
Daily self-monitoring of eating habits, physical activity, and weight is crucial for success. 1
Critical Pitfalls to Avoid
Do not assume "healthy eating" equals caloric deficit—many nutrient-dense foods (nuts, avocados, olive oil, whole grains) are calorie-dense and easily consumed in excess. 1 A food diary will likely reveal the problem.
Do not pursue extensive metabolic workup with normal TSH. Her thyroid function is adequate; TSH of 2.87 mIU/L does not suggest hypothyroidism or subclinical disease requiring treatment. 2
Do not recommend dietary supplements marketed for weight loss—they lack clear evidence of effectiveness. 1 Focus on documented caloric deficit instead.
Do not expect exercise alone to produce weight loss. 1 The exercise is valuable for health and weight maintenance, but caloric intake is the primary driver of weight change.
If Initial Approach Fails
After 3 months of documented caloric deficit (1,400-1,500 kcal/day with food logs) without weight loss:
Consider indirect calorimetry to measure actual resting metabolic rate, though this is rarely necessary. 2
Evaluate for medication-induced weight gain if she takes antidepressants, antipsychotics, beta-blockers, or other weight-promoting medications. 2
Consider FDA-approved weight loss medications (liraglutide 3.0 mg, semaglutide, tirzepatide) only if BMI ≥30 kg/m² (her BMI is approximately 30 kg/m²) and only as adjunct to continued lifestyle intervention, never as monotherapy. 1, 3 At her current weight and height, she just meets criteria for pharmacotherapy consideration.
The overwhelming likelihood is that meticulous food logging will reveal caloric intake exceeding her estimates, and creating a true 500-750 kcal/day deficit will produce the expected 1-1.5 lb/week weight loss. 1