Low Thyroid (Hypothyroidism)

Underactive thyroid that slows metabolism, causing fatigue, weight gain, and cognitive issues.

Overview

The thyroid gland sits at the base of your neck and produces hormones that set the metabolic rate of every cell. When thyroid output drops, everything slows down — metabolism, energy production, brain function, digestion, and body temperature regulation. The most important thing to understand: standard TSH testing alone often misses subclinical hypothyroidism and conversion problems. A full thyroid panel (TSH, Free T4, Free T3, Reverse T3, and antibodies) provides a complete picture.

Common Symptoms

Persistent fatigue and sluggishness

Unexplained weight gain or inability to lose weight

Feeling cold when others are comfortable

Constipation

Dry skin and brittle nails

Hair loss or thinning (especially the outer third of eyebrows)

Brain fog and poor memory

Depressed mood

Muscle aches and joint stiffness

Elevated cholesterol

Slow heart rate

Puffy face and swelling

Key Lab Markers

TSH

Optimal: 0.5-2.5 mIU/L

Standard range goes to 4.5, but optimal is under 2.5. Elevated TSH means the brain is working harder to stimulate a sluggish thyroid.

Free T4

Optimal: Upper half of range

T4 is the inactive storage form that must be converted to T3. Low Free T4 confirms hypothyroidism.

Free T3

Optimal: Upper half of range

T3 is the active thyroid hormone. Some patients have normal T4 but poor T3 conversion.

Reverse T3

Optimal: Below 15 ng/dL

Elevated rT3 can block T3 at the receptor level even with normal Free T3 levels.

TPO Antibodies

Optimal: Below 35 IU/mL

Positive antibodies indicate Hashimoto's thyroiditis (autoimmune thyroid disease).

Thyroglobulin Antibodies

Optimal: Below 1 IU/mL

Another marker for autoimmune thyroid disease, sometimes positive when TPO is negative.

Common Causes

1.Hashimoto's thyroiditis (autoimmune — accounts for 80%+ of hypothyroidism)
2.Iodine deficiency (rare in developed countries but still a global cause)
3.Thyroid surgery or radioactive iodine treatment
4.Medications (lithium, amiodarone, interferon)
5.Pituitary dysfunction (secondary hypothyroidism)
6.Selenium deficiency (selenium is required for T4 to T3 conversion)
7.Chronic stress (cortisol promotes reverse T3 production)
8.Gluten sensitivity (molecular mimicry with thyroid tissue in susceptible individuals)

Evidence-Based Management

1Thyroid hormone replacement (levothyroxine, liothyronine, or desiccated thyroid) under medical supervision

2Test a full thyroid panel, not just TSH — Free T3, Free T4, Reverse T3, and antibodies

3Optimize selenium (200 mcg/day) — essential for T4 to T3 conversion

4Ensure adequate iodine — but avoid excess, which can worsen Hashimoto's

5Address gut health — 20% of T4 to T3 conversion occurs in the gut

6Manage stress — cortisol promotes conversion of T4 to Reverse T3 instead of active T3

7Consider gluten elimination trial if antibodies are elevated

8Iron and ferritin optimization — iron is required for thyroid peroxidase enzyme function

Frequently Asked Questions

Why doesn't my doctor test Free T3?

Many doctors rely on TSH alone, which misses conversion problems (adequate T4 but poor T3 production) and receptor-level issues (high Reverse T3). You may need to specifically request a full thyroid panel including Free T3, Free T4, Reverse T3, and antibodies.

Can hypothyroidism be reversed?

It depends on the cause. Hashimoto's damage may be permanent, but reducing antibodies through diet and lifestyle can slow progression and improve function. Subclinical hypothyroidism from stress, nutrient deficiencies, or gut issues may improve significantly with targeted intervention.

Does hypothyroidism cause weight gain?

Yes, but typically 5-15 pounds rather than severe obesity. Hypothyroidism slows metabolism, promotes fluid retention, and can reduce motivation for exercise. Treating the thyroid and optimizing metabolism addresses the weight component.