What causes rapid weight gain at 37 despite eating less?

Question

Hello doctor,

I am 37 years old, 5 feet 6 inches tall (168 cm), and I weigh 187 pounds. I have been struggling with rapid weight gain for the past few years. Even when I eat very little or follow a diet, I still gain weight easily. This has been really frustrating, and I am hoping to understand what might be causing it.

Here are my recent lab results:

TSH: 5.65 µIU/mL.
Free T4: 0.98 ng/dL.
Free T3: 2.2 pg/mL.
Insulin (fasting): 25.4 µIU/mL.
Fasting glucose: 4.9 mmol/L (about 88 mg/dL)
HbA1c: 4.4 %.
Ferritin: 21.0 ng/mL.
Vitamin D (25-OH): 16.2 ng/mL.
Vitamin B12: 492 pg/mL.
Hemoglobin: 7.6 g/dL.
RBC: 3.5 million/µL.
Hematocrit: 30.6 %.
WBC: 8.5 x10³/µL.
Cortisol (morning): 187.4 nmol/L (approx. 6.8 µg/dL).
Estradiol (E2): 43.2 pg/mL.
Prolactin: 11.0 ng/mL.
Testosterone (total): 0.19 ng/mL.
ESR: 11 mm/hr.

I would really appreciate your help in reviewing these results and giving recommendations for what steps I can take next, whether that is medication, dietary changes, or further tests. Please help.

Thank you.

Answer 1

Hi,

Welcome to iclinic.com.

It is great to hear from you.

I can hear how frustrating it must be to feel like you are doing everything right, eating less, trying to take care of yourself, and still watching the weight go up. You are not imagining things. Your body is telling you something, and your lab results help us understand why.

Let us go through what I see:

Thyroid function: Your TSH (thyroid-stimulating hormone) is slightly high, and free T3 (triiodothyronine) is on the lower end, even though free T4 (thyroxine)is in range. This suggests your thyroid may not be working as efficiently as your body needs, a state we call subclinical hypothyroidism (happens when your thyroid gland is not making enough hormones). It is common in women and often overlooked, but it can quietly slow your metabolism, sap your energy, and make weight loss feel impossible.
Insulin resistance: Your insulin levels are quite elevated, even though your blood sugar looks fine. This means your body is working overtime to keep sugar in check. It is called insulin resistance, and it is a major reason why many women cannot lose weight, even with a perfect diet. This needs special attention.
Low iron and anemia: Your ferritin, hemoglobin, and RBC (red blood cells) levels are low, which points to iron deficiency anemia (where your body does not have enough healthy red blood cells to carry oxygen to your tissues). This alone can cause tiredness, weakness, and brain fog. It is not just about feeling "low". It is your body running on empty.
Vitamin D deficiency: Your vitamin D level is very low, which can affect mood, energy, immunity, and even weight gain.
Hormonal imbalances: Your estrogen and testosterone levels are on the lower side. These hormones play a big role in how you feel, how you store fat, and how your body maintains muscle.

What I would recommend (one step at a time):

Thyroid support: Talk to your doctor about possibly starting or adjusting a low-dose thyroid medication.
Boost your iron and vitamin D: Supplements will help, but your body also needs time and consistency.
Tackle insulin resistance: A nutrition plan that is low in refined carbs and sugar, and rich in fiber and protein, can help reset your insulin levels.
Hydration and rest: These may sound simple, but they make a big difference in recovery and hormone balance.

You are not alone in this. Your symptoms are real, and now you have a clearer path forward. You deserve to feel well in your body again, and that is absolutely possible with the right plan and care.

I hope this helps.

Kindly revert so I can assist you further.

Thank you.