Hair Loss and Your Thyroid: What You Need to Know

Your thyroid gland is a small, butterfly-shaped structure at the base of your neck that punches well above its weight. It produces hormones that regulate metabolism, energy, temperature, heart rate, and dozens of other functions. Among them: hair growth.

Both too little and too much thyroid hormone can disrupt the hair growth cycle and cause noticeable thinning. If you're losing hair and can't figure out why, your thyroid should be on the suspect list — especially if you're also experiencing fatigue, weight changes, or temperature sensitivity that you can't explain.

How Thyroid Hormones Affect Hair

Thyroid hormones (T3 and T4) directly influence hair follicle cycling. [Organ-culture work on isolated human scalp follicles](https://pubmed.ncbi.nlm.nih.gov/18728176/) showed that T3 and T4 prolong anagen, increase matrix keratinocyte proliferation, reduce keratinocyte apoptosis, and stimulate intrafollicular melanin synthesis — providing the mechanistic basis for the bullets below. They:

• Prolong the anagen (growth) phase — When thyroid hormones are at normal levels, they help keep hair in its active growth phase
• Stimulate hair matrix keratinocyte proliferation — The cells that actually build the hair shaft require thyroid hormone signaling to function properly
• Regulate pigmentation — Thyroid dysfunction can affect hair color, sometimes causing premature graying
• Affect the dermal papilla — The command center at the base of the hair follicle that orchestrates the growth cycle

When thyroid levels fall outside the normal range — in either direction — these processes get disrupted. The result is hair that enters the resting phase prematurely, sheds excessively, and may grow back thinner or more slowly.

Hypothyroidism and Hair Loss

Hypothyroidism — an underactive thyroid — is the more common thyroid condition, affecting about 5% of the US population, with subclinical hypothyroidism affecting up to 10%.

How It Causes Hair Loss

When thyroid hormone levels are too low:

• Hair follicles spend less time in the growth phase and more time in the resting (telogen) phase
• New hair growth slows, so lost hairs aren't replaced as quickly
• Hair becomes dry, coarse, and brittle
• The outer third of the eyebrows may thin (a classic hypothyroid sign)
• Shedding is typically diffuse — spread evenly across the scalp rather than concentrated in one area

What Hypothyroid Hair Loss Looks Like

• Gradual, diffuse thinning over the entire scalp
• Hair that feels dry, straw-like, or brittle
• Increased hair breakage (not just shedding from the root)
• Thinning eyebrows, particularly the lateral third
• Slow regrowth after cutting or styling
• Often accompanied by dry skin, fatigue, weight gain, constipation, and cold intolerance

Common Causes of Hypothyroidism

Hashimoto's thyroiditis is the most common cause in the US. It's an autoimmune condition where your immune system attacks the thyroid gland, gradually destroying its ability to produce hormones. It's far more common in women (7:1 ratio) and often runs in families.

Other causes include:

• Previous thyroid surgery or radioactive iodine treatment
• Certain medications (lithium, amiodarone)
• Iodine deficiency (rare in the US but common globally)
• Postpartum thyroiditis

Hashimoto's Thyroiditis: A Special Case for Hair Loss

Hashimoto's deserves its own section because it can affect hair through multiple mechanisms beyond just low thyroid hormone:

Autoimmune inflammation: The systemic inflammation associated with Hashimoto's can independently trigger telogen effluvium (excessive shedding). Even when thyroid hormone levels are being adequately replaced, ongoing autoimmune activity can keep hair in a disrupted cycle.

Nutrient depletion: Hashimoto's is associated with deficiencies in iron, vitamin D, zinc, and selenium — all of which are important for hair growth. Correcting these deficiencies alongside thyroid hormone replacement often produces better hair outcomes.

Comorbid autoimmune conditions: Hashimoto's increases the risk of alopecia areata (autoimmune patchy hair loss). If you have Hashimoto's and notice round, smooth patches of hair loss rather than diffuse thinning, alopecia areata should be evaluated.

Fluctuating levels: In early Hashimoto's, thyroid hormone levels can swing between hypo and hyperthyroid as the gland goes through cycles of destruction and hormone release (hashitoxicosis). These fluctuations can trigger repeated bouts of telogen effluvium.

Hyperthyroidism and Hair Loss

Hyperthyroidism — an overactive thyroid — is less common than hypothyroidism but can also cause significant hair loss.

How It Causes Hair Loss

Too much thyroid hormone:

• Accelerates the hair growth cycle too quickly, causing follicles to prematurely enter the catagen and telogen phases
• Results in finer, softer hair texture
• Can trigger diffuse thinning similar to hypothyroidism
• Hair may appear silky but fragile

What Hyperthyroid Hair Loss Looks Like

• Diffuse thinning across the scalp
• Hair that's fine, soft, and doesn't hold styles well
• Faster growth but increased shedding
• Often accompanied by weight loss, rapid heartbeat, anxiety, heat intolerance, tremor, and sweating

Common Causes of Hyperthyroidism

• Graves' disease — The most common cause, also autoimmune
• Toxic multinodular goiter
• Thyroiditis (inflammation causing temporary hormone release)
• Excessive thyroid medication (overtreated hypothyroidism)

Getting the Right Tests

If you suspect thyroid-related hair loss, the right blood work matters. A complete thyroid evaluation should include:

Essential Tests

• TSH (thyroid-stimulating hormone) — The primary screening test. Elevated in hypothyroidism, suppressed in hyperthyroidism. But TSH alone isn't always sufficient.
• Free T4 (free thyroxine) — The actual circulating thyroid hormone level. Important for confirming the diagnosis and monitoring treatment.
• Free T3 (free triiodothyronine) — The more active thyroid hormone. Some patients convert T4 to T3 poorly, and low T3 can cause symptoms even with normal TSH and T4.
• Thyroid antibodies (TPO and thyroglobulin antibodies) — Positive in Hashimoto's. Important for identifying the autoimmune component even when TSH is still in the normal range.

Additional Tests Worth Getting

• Ferritin — Iron stores are frequently low in thyroid patients, especially women. Low ferritin independently causes hair loss and compounds the thyroid effect. A [retrospective review of 2,851 women with telogen effluvium](https://pubmed.ncbi.nlm.nih.gov/39950230/) showed that low ferritin coexisted with abnormal TSH or T3 in a substantial subset, underscoring why a thyroid-only workup misses common contributors.
• Vitamin D — Deficiency is common in autoimmune thyroid disease and contributes to hair loss
• Zinc — Necessary for thyroid hormone synthesis and hair follicle function
• Selenium — Important for T4-to-T3 conversion and may reduce thyroid antibody levels

The "Normal" Range Problem

Here's something that catches a lot of patients: standard lab reference ranges for TSH are broad (typically 0.4-4.5 mIU/L). But many patients feel symptomatic — including experiencing hair loss — at TSH levels that are technically "normal" but suboptimal.

Many endocrinologists and functional medicine practitioners aim for a TSH of 0.5-2.5 mIU/L for symptomatic patients, particularly those with Hashimoto's. The conversation about optimal vs. normal is important, especially if your TSH is "normal" but you're still losing hair.

At CORAL, Dr. Kim orders comprehensive thyroid panels — not just TSH — and interprets them in the context of your symptoms, not just the reference range.

Treatment and Hair Recovery

Step 1: Optimize Thyroid Levels

For hypothyroidism, treatment is thyroid hormone replacement:

• Levothyroxine (Synthroid, Levoxyl) — The standard treatment. Synthetic T4 that your body converts to T3. Most patients do well on this.
• Liothyronine (Cytomel) — Synthetic T3. Sometimes added for patients who don't convert T4 to T3 efficiently or who remain symptomatic on levothyroxine alone.
• Combination T4/T3 therapy — Used when T4 alone isn't sufficient. Dosing requires careful titration.
• Natural desiccated thyroid (Armour, NP Thyroid) — Contains both T4 and T3 derived from porcine thyroid. Preferred by some patients who don't respond well to synthetic T4 alone.

For hyperthyroidism, treatment depends on the cause — antithyroid medications (methimazole), radioactive iodine, or surgery.

Key point: Hair recovery after thyroid optimization is slow. It takes 3-6 months after reaching stable, optimal thyroid levels before you'll notice improvement in hair growth. The hair growth cycle is inherently slow, and follicles need time to reset.

Step 2: Address Nutrient Deficiencies

Even with optimal thyroid levels, hair won't recover well if key nutrients are missing:

• Iron (ferritin target: 50-70+ ng/mL) — Supplement if low. Iron bisglycinate is generally better tolerated than ferrous sulfate.
• Vitamin D (target: 40-60 ng/mL) — Supplement as needed, typically 2,000-5,000 IU daily
• Zinc (15-30mg daily) — If levels are low. Don't oversupplement — excess zinc can cause copper deficiency.
• Selenium (200mcg daily) — May help reduce TPO antibodies in Hashimoto's. Don't exceed 400mcg. A [2025 systematic review and meta-analysis of 21 randomized trials](https://pubmed.ncbi.nlm.nih.gov/40898469/) found that selenium supplementation significantly lowered TPO antibody levels at 3 and 6 months in Hashimoto's patients.

Step 3: Consider Hair-Specific Treatments

If thyroid optimization and nutrient correction don't fully resolve the hair loss after 6-12 months, additional treatments may be needed:

• Minoxidil (topical or oral) — Can accelerate recovery and is appropriate as an adjunct
• Evaluation for concurrent FPHL or MPHL — Thyroid-related hair loss and genetic pattern hair loss can coexist, and each requires its own treatment
• Anti-inflammatory support — For Hashimoto's patients, reducing autoimmune inflammation may help (this is where selenium, vitamin D optimization, and stress management play a role)

Step 4: Be Patient

This is perhaps the hardest part. Hair follicles operate on their own timeline. Even with perfect treatment:

• Initial improvement in shedding: 6-8 weeks after reaching optimal levels
• Visible new growth: 3-6 months
• Significant density improvement: 6-12 months
• Full recovery: 12-18 months in some cases

The hair that grows back may initially be finer or lighter. This is normal — the follicles are recovering and will produce thicker, more pigmented hairs over subsequent growth cycles.

When to Seek Help

Get your thyroid checked if you notice:

• Diffuse hair thinning without an obvious cause
• Thinning of the outer eyebrows
• Hair that's become dry, brittle, or changed texture
• Hair loss accompanied by fatigue, weight changes, or temperature sensitivity
• Family history of thyroid disease (especially Hashimoto's)
• Postpartum hair loss that seems excessive or prolonged

The connection between thyroid function and hair health is direct and treatable. But you need the right tests, the right interpretation, and a treatment plan that addresses all contributing factors — not just TSH. Dr. Kim provides comprehensive thyroid evaluations through telehealth, including labs, interpretation, and treatment management. Start at [coral.clinic/start](https://coral.clinic/start).