Thyroid Dysfunction
Also known as: Hypothyroidism / Subclinical Thyroid Disease
The thyroid gland regulates metabolism, energy production, body temperature, heart rate, digestion, mood, and cognitive function. When it underperforms (hypothyroidism) or overperforms (hyperthyroidism), the effects cascade across every organ system. An estimated 20 million Americans have some form of thyroid disease, and up to 60% are unaware of their condition.
In this article
What Is Thyroid Dysfunction?
The thyroid produces two primary hormones: T4 (thyroxine, the storage form) and T3 (triiodothyronine, the active form). The pituitary gland regulates thyroid output via TSH (thyroid-stimulating hormone). When thyroid hormones are low, TSH rises; when adequate, TSH decreases.
Types of Thyroid Dysfunction
- Overt hypothyroidism: TSH elevated above range with low Free T4. Clear clinical symptoms. Affects approximately 5% of the population.
- Subclinical hypothyroidism: TSH elevated (2.5-10 mIU/L) with normal Free T4. Symptoms present but often dismissed as “normal.” Affects up to 10% of women over 40.
- Poor T4-to-T3 conversion: TSH and T4 may appear normal, but Free T3 is low. The body produces thyroid hormone but cannot convert it to the active form. Often missed when only TSH is tested.
- Hashimoto's thyroiditis: Autoimmune destruction of the thyroid gland. The most common cause of hypothyroidism. Diagnosed by elevated TPO or thyroglobulin antibodies.
Symptoms
Hypothyroidism Symptoms
- Fatigue and low energy (the most common complaint)
- Weight gain or inability to lose weight
- Cold intolerance (always feeling cold)
- Constipation
- Dry skin and brittle nails
- Hair thinning or loss (including outer third of eyebrows)
- Brain fog and poor concentration
- Depression
- Muscle weakness and joint pain
- Elevated cholesterol
- Slow heart rate
- Irregular or heavy menstrual periods
Key insight: Many of these symptoms overlap with perimenopause, low testosterone, and depression. This is why comprehensive testing — not just TSH — is essential for accurate diagnosis.
How Thyroid Interacts with Other Hormones
- Estrogen: Increases thyroid-binding globulin, reducing free thyroid hormones. This is why perimenopausal women often develop thyroid symptoms as estrogen fluctuates.
- Progesterone: Enhances thyroid hormone sensitivity at the cellular level. Low progesterone reduces effective thyroid activity.
- Cortisol: Chronic stress inhibits TSH release and impairs T4-to-T3 conversion, creating functional hypothyroidism even with a healthy thyroid gland.
- Testosterone: Low thyroid function alters SHBG production, affecting free testosterone levels in both men and women.
Optimizing thyroid function often requires addressing the broader hormonal environment — not just prescribing levothyroxine.
Diagnosis & Testing
A complete thyroid assessment requires more than TSH alone:
- TSH: Standard range 0.45-4.5 mIU/L. Optimal: 0.5-2.0 mIU/L.
- Free T4: Storage hormone. Should be in the upper half of the reference range.
- Free T3: Active hormone. The most functionally relevant marker. Optimal: 3.0-4.0 pg/mL.
- Reverse T3: Inactive form. Elevated levels indicate impaired conversion (often stress-related).
- Thyroid antibodies (TPO, TgAb): Should be tested at least once to rule out autoimmune thyroiditis.
If your doctor only tests TSH, you are getting an incomplete picture. Many patients with “normal” TSH have low Free T3, elevated Reverse T3, or elevated antibodies that explain their symptoms.
Treatment
- Levothyroxine (T4): Standard first-line treatment. Effective for many patients, but some do not convert T4 to T3 efficiently.
- Combination T4/T3 therapy: Adding liothyronine (T3) or using desiccated thyroid (contains both T4 and T3) can help patients who do not respond adequately to T4 alone.
- Nutritional support: Selenium (T4-to-T3 conversion), zinc (thyroid hormone production), iron (thyroid peroxidase function), vitamin D (autoimmune protection).
- Address the hormonal environment: Optimizing cortisol, estrogen, and progesterone often improves thyroid function without changing thyroid medication.
Related Conditions
Medical disclaimer: This article is for informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay seeking it because of something you have read on this website.
Medically reviewed. Last updated: March 2026.