Thyroid and Weight Gain: When Your Hormones Are the Real Problem

You've gained weight. Not from any obvious change in diet or exercise — it just crept on. You're tired all the time, your brain feels like it's running through fog, and you're cold when everyone else is comfortable. You mention it to your doctor. They check your TSH. It comes back "normal." Case closed.

Except you still feel terrible.

This scenario plays out millions of times a year, and it represents one of the most frustrating gaps between how thyroid dysfunction actually works and how it's commonly evaluated. If your only thyroid test has been TSH, you may have an incomplete picture — and that incomplete picture might be the reason you're still struggling with weight, fatigue, and feeling like yourself.

How the Thyroid Controls Your Metabolism

Your thyroid gland, a butterfly-shaped organ at the base of your neck, is essentially your body's metabolic thermostat. It produces two primary hormones:

T4 (thyroxine) — The storage form of thyroid hormone. Your thyroid produces about 80% T4 and 20% T3. T4 itself is relatively inactive — it must be converted to T3 to do its job.

T3 (triiodothyronine) — The active form. T3 enters your cells and directly influences metabolic rate, body temperature, heart rate, energy production, and virtually every tissue in your body.

The feedback loop: The hypothalamus releases TRH, which tells the pituitary to release TSH, which tells the thyroid to produce T4 and T3. When T4/T3 levels are adequate, TSH goes down. When they're low, TSH goes up.

Why this matters for weight: Thyroid hormones regulate your basal metabolic rate — the number of calories your body burns at rest. When thyroid hormone levels are low, your metabolic rate drops, sometimes significantly. You burn fewer calories doing everything, and the surplus gets stored as fat. Additionally, hypothyroidism causes fluid retention, which can add 5-10 pounds independent of actual fat gain.

Why TSH Alone Isn't Enough

TSH is useful. It's the most sensitive screening test for primary thyroid dysfunction, and in many cases, it's all that's needed. But there are specific scenarios where normal TSH doesn't tell the whole story:

The Normal Range Debate

Most labs define the normal TSH range as approximately 0.4-4.5 mIU/L. But this range is based on population data that includes people with subclinical thyroid dysfunction. Many endocrinologists and functional medicine practitioners argue that the optimal range is narrower — typically 0.5-2.5 mIU/L.

A TSH of 3.8 is "normal" by lab standards but might represent suboptimal thyroid function for a given individual. Some people feel their best at TSH levels between 1.0-2.0 and experience symptoms at levels that are technically within range.

Conversion Problems

Even with adequate TSH and T4, some people don't efficiently convert T4 to T3. This means the active hormone their cells need isn't available in adequate amounts, despite the screening test looking normal.

Factors that impair T4-to-T3 conversion:

• Selenium deficiency — Selenium is a cofactor for the deiodinase enzymes that convert T4 to T3
• Zinc and iron deficiency — Both support conversion
• Chronic stress — Cortisol can shift conversion toward reverse T3 (rT3), an inactive form that blocks T3 receptors
• Inflammation — Chronic inflammation impairs deiodinase activity
• Caloric restriction — Your body reduces T3 production during energy deficit as a survival mechanism
• Certain medications — Beta-blockers, amiodarone, and lithium can affect conversion

Hashimoto's Thyroiditis

Hashimoto's is an autoimmune condition where the immune system attacks the thyroid gland. It's the most common cause of hypothyroidism in the United States and affects women about 7 times more often than men.

The tricky part: Hashimoto's can cause symptoms for years before TSH becomes abnormal. In the early stages, the thyroid is under attack but compensating by working harder. TSH stays normal, but symptoms are present — and thyroid antibodies (TPO and thyroglobulin antibodies) may be elevated.

If your doctor only checks TSH and it's normal, Hashimoto's in its early stages can be missed entirely.

The Complete Thyroid Panel

At CORAL, Dr. Kim orders a comprehensive thyroid panel rather than TSH alone:

• TSH — Screening and monitoring
• Free T4 — Unbound, available T4 (more useful than total T4)
• Free T3 — The active hormone. Low free T3 with normal TSH suggests a conversion issue
• Reverse T3 (rT3) — An inactive metabolite that can block T3 at the receptor level. Elevated rT3 can explain hypothyroid symptoms despite normal-looking conventional labs
• TPO antibodies — Elevated in Hashimoto's thyroiditis. Present in 90-95% of Hashimoto's cases
• Thyroglobulin antibodies — Another marker for Hashimoto's, present in about 60-70% of cases

Additional labs to consider:

• Ferritin — Iron deficiency impairs both thyroid hormone production and conversion
• Vitamin D — Deficiency is associated with increased Hashimoto's risk
• Selenium — Low levels impair T4-to-T3 conversion
• Cortisol — Chronic stress affects thyroid function

Hashimoto's: The Autoimmune Connection

Hashimoto's deserves special attention because it's common, often underdiagnosed, and has implications beyond thyroid hormone levels:

What's happening: Your immune system produces antibodies against thyroid peroxidase (TPO) and/or thyroglobulin — proteins essential for thyroid hormone production. These antibodies cause chronic inflammation of the thyroid gland, leading to progressive destruction and eventual hypothyroidism.

The progression:

1. Subclinical phase — Antibodies are present, thyroid is under attack, but TSH and thyroid hormones are still normal. Symptoms may already be present.
2. Subclinical hypothyroidism — TSH starts to rise (typically 5-10), but free T4 and T3 remain in the normal range. This is where the treatment debate begins.
3. Overt hypothyroidism — TSH is elevated, and T4/T3 are low. Symptoms are typically significant.

Managing Hashimoto's beyond medication:

• Gluten evaluation — There's a well-documented association between celiac disease and Hashimoto's. Even without celiac disease, some Hashimoto's patients see antibody reduction with gluten elimination, though this isn't universal
• Selenium supplementation — 200 mcg daily has been shown in multiple studies to reduce TPO antibodies in Hashimoto's patients
• Anti-inflammatory diet — Reducing inflammatory triggers can modulate the autoimmune process
• Gut health — Intestinal permeability ("leaky gut") may contribute to autoimmune thyroid disease
• Stress management — Chronic stress exacerbates autoimmune conditions

Treatment Options

Levothyroxine (T4 Only)

The standard of care. Levothyroxine (Synthroid, Levoxyl, generic) provides synthetic T4, which your body converts to T3.

• Advantages: Well-studied, consistent dosing, long half-life (7 days), inexpensive
• Limitations: Depends on adequate conversion to T3. Patients with conversion issues may not feel optimal on T4 alone
• Dosing: Typically starts at 25-50 mcg and increases by 25 mcg every 6-8 weeks until TSH is optimized
• Important: Take on an empty stomach, 30-60 minutes before food. Avoid taking with calcium, iron, or coffee (all reduce absorption)

Combination T4/T3 Therapy

For patients who don't feel optimal on T4 alone, adding T3 (liothyronine/Cytomel) can make a significant difference.

• When to consider: Normal TSH and free T4 on levothyroxine but persistent symptoms, low free T3, or elevated rT3
• Approach: Small amounts of T3 (5-10 mcg daily, often split into morning and afternoon doses) added to existing T4 therapy
• Cautions: T3 has a short half-life and can cause heart palpitations, anxiety, and insomnia if dosed too aggressively. Start low and increase slowly

Natural Desiccated Thyroid (NDT)

Medications like Armour Thyroid, NP Thyroid, and WP Thyroid contain both T4 and T3 derived from porcine (pig) thyroid glands.

• Advantages: Contains both T4 and T3 in a fixed ratio, along with small amounts of T1, T2, and calcitonin
• Limitations: The T4:T3 ratio in NDT (approximately 4:1) doesn't match the human ratio (approximately 14:1), which means NDT provides relatively more T3 than the body naturally produces
• Patient preference: Some patients report feeling significantly better on NDT compared to levothyroxine alone. This is clinically observed even when the studies are mixed
• Monitoring: Requires the same lab monitoring as synthetic options, with special attention to free T3 levels

The Weight Connection: Setting Realistic Expectations

Here's where honesty matters: treating hypothyroidism will help with weight, but it's not a magic solution.

What thyroid treatment typically does for weight:

• Resolves fluid retention (often the first 5-10 pounds)
• Normalizes metabolic rate (allows your body to burn calories at the rate it should)
• Improves energy and motivation (making exercise and healthy eating more feasible)
• Typical weight loss from treating hypothyroidism: 5-15 pounds, mostly from fluid and metabolic normalization

What thyroid treatment doesn't do:

• Reverse years of metabolic adaptation
• Overcome poor dietary habits
• Compensate for insulin resistance or other metabolic conditions
• Guarantee you'll reach your ideal weight

If you've gained significant weight and your thyroid is the only thing being treated, it may be necessary to address other factors: insulin resistance, cortisol, sex hormones, dietary patterns, and exercise habits.

Getting a Complete Evaluation

If you've been told your thyroid is "fine" based on TSH alone but you're still struggling with weight gain, fatigue, brain fog, cold intolerance, or hair loss, a complete thyroid evaluation is worth pursuing.

Start at [coral.clinic/start](https://coral.clinic/start). Dr. Kim orders the full panel — not just TSH — and interprets the results in the context of your symptoms, not just reference ranges. Because "normal" lab values in a person who feels terrible aren't really normal.

Your thyroid might be working harder than you think — and not hard enough.