Both hypothyroidism and hyperthyroidism cause diffuse hair loss by disrupting the growth cycle of hair follicles. Thyroid-related hair loss is reversible in most cases once hormone levels are corrected with medication, with regrowth typically beginning within 3 to 6 months of reaching stable thyroid function.
This article is for informational purposes only and does not constitute medical advice.
How Thyroid Hormones Affect Hair
Thyroid hormones (T3 and T4) regulate metabolism in virtually every cell in the body, including hair follicle cells. These hormones directly influence the rate of cell division in the hair matrix, the duration of the growth phase (anagen), and the transition between growth and resting phases.
When thyroid hormone levels fall outside the normal range in either direction, hair follicles respond by shortening the growth phase and prematurely entering the resting phase (telogen). The result is telogen effluvium: diffuse, widespread shedding that affects the entire scalp.
Hypothyroidism vs Hyperthyroidism: Hair Effects
| Factor | Hypothyroidism | Hyperthyroidism |
|---|---|---|
| Thyroid function | Underactive (too little T3/T4) | Overactive (too much T3/T4) |
| Hair loss pattern | Diffuse thinning, slow onset | Diffuse thinning, can be rapid |
| Hair texture change | Dry, coarse, brittle | Fine, soft, fragile |
| Eyebrow loss | Outer third (classic sign) | Less common |
| Other symptoms | Fatigue, weight gain, cold sensitivity | Weight loss, anxiety, heat sensitivity |
| Prevalence | More common (5% of population) | Less common (1-2% of population) |
The Eyebrow Sign
Loss of hair in the outer third of the eyebrows is a well-known clinical indicator of hypothyroidism. While not present in every case, it is specific enough that dermatologists routinely check for it during hair loss evaluations. If you have noticed eyebrow thinning alongside scalp hair loss, thyroid testing should be a priority.
Testing for Thyroid Dysfunction
Essential Blood Tests
| Test | What It Measures | Normal Range |
|---|---|---|
| TSH | Thyroid-stimulating hormone (primary screening test) | 0.4 to 4.0 mIU/L |
| Free T4 | Active thyroxine | 0.8 to 1.8 ng/dL |
| Free T3 | Active triiodothyronine | 2.3 to 4.2 pg/mL |
| TPO antibodies | Autoimmune thyroid disease marker | Below 35 IU/mL |
TSH is the standard screening test. An elevated TSH suggests hypothyroidism; a suppressed TSH suggests hyperthyroidism. However, TSH alone can miss subclinical thyroid dysfunction. For a complete picture, especially when hair loss is the primary complaint, request all four tests.
Subclinical Thyroid Dysfunction and Hair
Some patients have TSH levels at the edges of the normal range (for example, TSH of 3.5 to 4.0) with normal T3 and T4. This is called subclinical hypothyroidism. Research suggests that even subclinical thyroid dysfunction can contribute to hair thinning, though the effect is less dramatic than overt hypothyroidism.
| Thyroid Status | TSH Level | Likely Hair Impact |
|---|---|---|
| Overt hypothyroidism | Above 10 mIU/L | Significant hair loss, texture changes |
| Subclinical hypothyroidism | 4.0 to 10 mIU/L | Mild thinning possible |
| Optimal | 1.0 to 2.5 mIU/L | Best environment for hair growth |
| Subclinical hyperthyroidism | 0.1 to 0.4 mIU/L | Mild thinning possible |
| Overt hyperthyroidism | Below 0.1 mIU/L | Significant hair loss, texture changes |
Treatment and Recovery
For Hypothyroidism
The standard treatment is levothyroxine (Synthroid, Levoxyl), a synthetic T4 hormone taken daily. Once dosing is optimized (usually within 6 to 8 weeks of adjustments), thyroid hormones stabilize and hair follicles gradually return to their normal growth cycle.
For Hyperthyroidism
Treatment options include anti-thyroid medications (methimazole, propylthiouracil), radioactive iodine ablation, or surgery. The goal is to bring T3 and T4 back into the normal range. Hair recovery follows thyroid stabilization.
Hair Regrowth Timeline After Treatment
| Milestone | Expected Timeframe |
|---|---|
| Thyroid levels stabilize | 6 to 12 weeks after starting treatment |
| Shedding slows | 2 to 4 months after levels stabilize |
| New growth visible | 3 to 6 months after levels stabilize |
| Noticeable improvement | 6 to 9 months |
| Full recovery | 9 to 18 months |
Hair recovery from thyroid dysfunction takes patience. The hair growth cycle is inherently slow, and follicles that were pushed into telogen need to complete the resting phase before new growth begins.
Thyroid Hair Loss vs Other Types
It is important to distinguish thyroid-related hair loss from androgenetic alopecia (pattern baldness) because the treatments are completely different. Use our Norwood scale guide to check whether your loss follows a pattern.
How to Tell the Difference
| Indicator | Thyroid Hair Loss | Androgenetic Alopecia |
|---|---|---|
| Pattern | Diffuse, entire scalp | Temples, crown, frontal hairline |
| Onset | Coincides with thyroid symptoms | Gradual, over years |
| Other symptoms | Fatigue, weight changes, temperature sensitivity | None |
| Eyebrow involvement | Often, especially outer third | Rarely |
| Reversibility | Yes, with thyroid treatment | Progressive without DHT blockers |
| Gender predominance | Women (5:1 ratio) | Men (though women also affected) |
Thyroid dysfunction and androgenetic alopecia can co-exist. A patient may have genetic pattern baldness worsened by untreated hypothyroidism. Treating the thyroid issue will recover the diffuse component of the loss, but the patterned component will require separate intervention.
Thyroid patients commonly also have iron deficiency, particularly women with hypothyroidism. Read our guide on iron deficiency and hair loss to understand the overlap and ensure both are addressed.
Next Step
Wondering whether your hair loss is thyroid-related or patterned? Upload a photo at myhairline.ai/analyze to get a pattern analysis that distinguishes between diffuse shedding and androgenetic alopecia.