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High Prevalence — Often Undiagnosed
Hypothyroidism affects approximately 10% of women and 2–3% of men over 60. In the United States, it is estimated that 1 in 8 women will develop thyroid disease over their lifetime. Most cases are caused by Hashimoto's thyroiditis — a silent autoimmune disease — and diagnosis is often made only when symptoms already significantly impact quality of life.
1. The Thyroid and the Hypothalamus-Pituitary-Thyroid (HPT) Axis
The thyroid is a butterfly-shaped gland located at the base of the neck. It produces two main hormones: T4 (thyroxine) and T3 (triiodothyronine) — with T3 being the biologically active form (T4 is converted to T3 by peripheral tissues via deiodinases).
Hormone production is controlled via the HPT axis: the hypothalamus releases TRH (thyrotropin-releasing hormone), which stimulates the pituitary to produce TSH (thyroid-stimulating hormone). TSH in turn stimulates the thyroid to produce T4 and T3. When thyroid hormones fall (hypothyroidism), TSH rises — hence its use as the primary diagnostic marker.
2. Causes and Prevalence
The most common cause of hypothyroidism in the Western world is Hashimoto's thyroiditis (chronic autoimmune lymphocytic thyroiditis). In the rest of the world, iodine deficiency is still the main cause. Other causes include:
- Thyroidectomy (surgical removal of the thyroid) — permanent hypothyroidism
- Radioactive iodine treatment (for hyperthyroidism or cancer)
- Medications: lithium, amiodarone, interferon, checkpoint inhibitors
- Central hypothyroidism: pituitary or hypothalamic lesion (rare)
- Congenital hypothyroidism: identified by newborn screening
3. Symptoms: The Disease of Many Faces
Thyroid hormones regulate virtually all metabolic processes in the body. When lacking, symptoms are varied, insidious and frequently confused with depression, anemia or normal aging:
- Persistent fatigue and excessive drowsiness
- Weight gain without dietary changes (fluid retention + slow metabolism)
- Cold intolerance — constant feeling of being cold
- Constipation — slowed intestinal transit
- Dry skin, brittle hair and hair loss
- Cognitive slowing, difficulty concentrating ("brain fog")
- Depression — hypothyroidism is a reversible cause of depression
- Menstrual irregularity and infertility in women
- Bradycardia (slow heart rate) and dyslipidemia (elevated LDL)
- Facial swelling (myxedema) in severe cases
4. Diagnosis: TSH, Free T4 and Antibodies
The diagnosis of hypothyroidism is laboratory-based, primarily based on serum TSH. Screening is recommended for all adults over 35 every 5 years, and more frequently in women over 50.
| Condition | TSH (mIU/L) | Free T4 (ng/dL) | Interpretation |
|---|---|---|---|
| Normal | 0.5 – 4.5 | 0.8 – 1.8 | Normal thyroid function |
| Subclinical Hypothyroidism | 4.5 – 10.0 | Normal | Elevated TSH; T4 still compensated |
| Overt Hypothyroidism | > 10.0 | Low (< 0.8) | Treatment always indicated |
| Central Hypothyroidism | Normal or low | Low | Pituitary lesion; TSH does not reflect dysfunction |
| Hyperthyroidism | < 0.1 | Elevated (> 1.8) | Excessive hormone production |
| Subclinical Hyperthyroidism | 0.1 – 0.5 | Normal | Suppressed TSH; risk of AF and osteoporosis |
Anti-TPO antibodies (thyroid peroxidase) and anti-Tg (thyroglobulin) confirm the autoimmune etiology (Hashimoto's) and predict progression to overt hypothyroidism in subclinical cases.
5. Subclinical Hypothyroidism: To Treat or Not?
This is one of the most debated questions in endocrinology. The TRUST study (2017, NEJM) — the largest RCT on the subject — evaluated 737 elderly patients with subclinical hypothyroidism and found no significant benefit of levothyroxine on symptoms or quality of life in this population.
The current guideline (ATA 2014) recommends treatment when:
- TSH > 10 mIU/L (regardless of symptoms)
- TSH 4.5–10 mIU/L with attributable symptoms of hypothyroidism
- Pregnancy or trying to conceive (TSH > 2.5 already indicates treatment)
- Young patients (< 65 years) with TSH > 7 mIU/L
6. Hashimoto's Thyroiditis: The Autoimmune Cause
In Hashimoto's thyroiditis, the immune system produces antibodies that attack the thyroid, causing chronic inflammation and progressive destruction of the glandular parenchyma. It is the most common autoimmune disease in the world.
Interestingly, early stages of Hashimoto's can present with transient hyperthyroidism (Hashitoxicosis) — due to the release of stored hormones from the inflamed gland — before progressing to hypothyroidism.
Hashimoto's is associated with other autoimmune diseases: Type 1 Diabetes, Celiac Disease, Vitiligo, Sjögren's Syndrome and Lupus. Screening for these comorbidities is part of management.
7. Treatment: Levothyroxine — Dose, Timing and Absorption
Levothyroxine (L-T4) is the gold standard treatment. It is a synthetic hormone identical to human T4, low-cost and highly effective. The initial dose is individualized (typically 1.6 mcg/kg/day), with adjustments every 6–8 weeks based on TSH.
Levothyroxine Absorption — Critical Rules
- Take on an empty stomach, 30–60 minutes before breakfast (increases absorption by 40%).
- Do not take with coffee — caffeine reduces absorption by up to 26% even without food.
- Avoid calcium, iron, antacids with aluminum/magnesium and omeprazole for at least 4h after the dose.
- Soy, fiber and nuts can reduce absorption — maintain a 2–4h interval.
- Alternative: take at bedtime (>2h after last meal) — study showed improved TSH control.
8. Diet, Iodine and Supplements for Thyroid Health
Iodine is essential for thyroid hormone synthesis — each T4 molecule contains 4 iodine atoms. The recommended daily intake is 150 mcg (200 mcg in pregnancy). Iodized salt is the main dietary source.
Important: both iodine deficiency and excess can cause thyroid dysfunction. High-dose iodine supplements (above 500 mcg/day) can precipitate Hashimoto's or hyperthyroidism in predisposed individuals.
Selenium (200 mcg/day via selenomethionine) has been shown to reduce anti-TPO antibodies in randomized studies and may be considered as an adjunct in Hashimoto's. Zinc also participates in T4→T3 conversion.
References
1. Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism. Thyroid. 2014;24(12):1670-1751. (ATA 2014)
2. Garber JR, et al. Clinical Practice Guidelines for Hypothyroidism in Adults. Endocr Pract. 2012;18(6):988-1028. (AACE)
3. Stott DJ, et al. Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism (TRUST). N Engl J Med. 2017;376:2534-2544.
4. Biondi B, Cooper DS. The clinical significance of subclinical thyroid dysfunction. Endocr Rev. 2008;29(1):76-131.
5. Wiersinga WM, et al. 2012 ETA Guidelines: The Use of L-T4 + L-T3 in the Treatment of Hypothyroidism. Eur Thyroid J. 2012;1(1):55-71.
6. Surks MI, et al. Subclinical thyroid disease: scientific review and guidelines for diagnosis and management. JAMA. 2004;291(2):228-238.
7. Virili C, et al. Individualized Levothyroxine Therapy: Current Knowledge and Future Perspectives. Front Endocrinol. 2019;10:349.
8. Ventura M, et al. Selenium and Thyroid Disease: From Pathophysiology to Treatment. Int J Endocrinol. 2017;2017:1297658.