Key Points
- Primary hypothyroidism: elevated TSH + low free T4.
- Hashimoto thyroiditis is the most common cause in iodine-sufficient regions.
- Levothyroxine is first-line therapy; titrate using TSH every 6–8 weeks.
- Start low-dose therapy in older adults or those with cardiac disease.
- Review medication interactions that impair absorption.
Algorithm
- Assess symptoms and risk factors; obtain TSH ± free T4.
- If TSH elevated and free T4 low → diagnose primary hypothyroidism.
- Check TPO antibodies if autoimmune suspicion or subclinical disease.
- Start levothyroxine (dose adjusted by age, weight, cardiac history).
- Recheck TSH in 6–8 weeks; adjust dose.
- Establish long-term monitoring every 6–12 months once stable.
Clinical Synopsis & Reasoning
Primary hypothyroidism results from thyroid gland failure, most commonly autoimmune (Hashimoto thyroiditis). Patients present with nonspecific symptoms including fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, menstrual changes, and cognitive slowing. Physical findings may include bradycardia, diastolic hypertension, delayed reflex relaxation, and goiter. Diagnosis relies on elevated TSH with low free T4. Evaluate for autoimmune thyroiditis with TPO antibodies when appropriate. Consider secondary causes when TSH is low/normal with low free T4.
Treatment Strategy & Disposition
Levothyroxine is the treatment of choice. In healthy adults <60 years, start ~1.6 mcg/kg/day; use lower starting doses (e.g., 25–50 mcg/day) in older adults or those with cardiovascular disease. Recheck TSH 6–8 weeks after dose changes. Target TSH within reference range (typically 0.4–4.0 mIU/L). Counsel patients to take levothyroxine on an empty stomach, separated from calcium, iron, fiber supplements, and certain medications. Consider endocrinology referral for refractory symptoms, pregnancy, pituitary disease, or complex comorbidities.
Epidemiology / Risk Factors
- Prevalence increases with age and is more common in women.
- Autoimmune thyroid disease strongly associated with other autoimmune disorders, including type 1 diabetes and celiac disease.
Investigations
| Test | Role / Rationale | Typical Findings | Notes |
|---|---|---|---|
| TSH | Primary screening and monitoring | High | Hallmark of primary hypothyroidism |
| Free T4 | Confirmatory test | Low | Indicates hormone deficiency |
| TPO antibodies | Identify autoimmune thyroiditis | Positive in Hashimoto | Useful in subclinical disease |
| Lipid panel | Assess metabolic impact | Elevated LDL | Normalizes with treatment |
| CBC | Screen for normocytic anemia | Mild anemia | Reversible with therapy |
Common Causes of Primary Hypothyroidism
| Cause | Details |
|---|---|
| Hashimoto thyroiditis | Most common cause; autoimmune destruction of thyroid |
| Iatrogenic | Post-thyroidectomy, radioiodine therapy, radiation |
| Drugs | Amiodarone, lithium, interferon-α, TKIs |
| Iodine deficiency/excess | Worldwide leading cause (deficiency) |
Pharmacology
| Medication | Mechanism | Onset | Role in Therapy | Limitations |
|---|---|---|---|---|
| Levothyroxine (T4) | Synthetic thyroxine replacing deficient hormone | Weeks | First-line replacement therapy | Absorption affected by food, calcium, iron; requires titration |
| Liothyronine (T3) | Synthetic T3 providing rapid onset | Days | Rarely used; select cases only | Short half-life; risk of arrhythmias; not first-line |
Prognosis / Complications
- Excellent prognosis with appropriate replacement therapy.
- Untreated severe hypothyroidism can progress to myxedema coma, requiring emergent treatment.
- Long-term risks relate to under- or overtreatment (dyslipidemia, cardiovascular disease, bone loss).
Patient Education / Counseling
- Explain chronic nature of hypothyroidism and importance of consistent medication timing.
- Review supplements or medications that interfere with hormone absorption.
- Discuss dose adjustments during pregnancy, weight changes, or new medications.
Notes
Primary hypothyroidism requires lifelong hormone replacement in most cases. Ensuring proper levothyroxine administration is essential for stable thyroid levels. Consider alternative formulations (liquid or soft-gel levothyroxine) for patients with absorption issues or persistent TSH elevation despite adherence.
References
- Jonklaas J et al. Guidelines for the Treatment of Hypothyroidism: American Thyroid Association (2014). — Link
- Garber JR et al. Clinical Practice Guidelines for Hypothyroidism in Adults (AACE/ACE, 2012). — Link
- Vanderpump MPJ. The epidemiology of thyroid disease. Br Med Bull. 2011;99:39–51. — Link
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