Hyperthyroidism and Other Causes of Thyrotoxicosis¶
Chapter 396 | Part 12: Endocrinology and Metabolism
KEY CLINICAL POINTS¶
- Thyrotoxicosis is defined as excess thyroid hormone, distinct from hyperthyroidism (excessive thyroid function).
- Graves’ disease accounts for 60–80% of thyrotoxicosis cases, with peak incidence in 20–50-year-olds.
- Antithyroid drugs (e.g., methimazole, propylthiouracil) are first-line therapy, while radioiodine or surgery are alternatives.
- Pregnancy requires careful thyroid hormone monitoring and dose adjustments to avoid fetal hypothyroidism.
- Thyroid-associated ophthalmopathy (Graves’ ophthalmopathy) is managed with corticosteroids, orbital decompression, and teprotumumab.
1. DEFINITION & OVERVIEW¶
Thyrotoxicosis refers to the state of excess thyroid hormone, distinct from hyperthyroidism (excessive thyroid function). Causes include Graves’ disease, toxic multinodular goiter, toxic adenoma, and non-autoimmune causes like subacute thyroiditis or iodine excess. Myxedema coma is a severe complication with 20–40% mortality.
Table 396-1: Causes of Thyrotoxicosis¶
| Primary Hyperthyroidism | Secondary Hyperthyroidism | Other Causes |
|---|---|---|
| Graves’ disease | TSH-secreting pituitary adenoma | Subacute thyroiditis |
| Toxic multinodular goiter | Thyroid hormone resistance syndrome | Silent thyroiditis |
| Toxic adenoma | Chorionic gonadotropin-secreting tumors | Iodine excess (Jod-Basedow phenomenon) |
| Functioning thyroid carcinoma metastases | Gestational thyrotoxicosis | Thyrotoxicosis factitia (excess thyroid hormone ingestion) |
1.1 Thyroid Hormone Excess¶
Thyrotoxicosis is not synonymous with hyperthyroidism. It encompasses all conditions where thyroid hormone levels are elevated, including non-autoimmune causes like destructive thyroiditis or iodine excess.
1.2 Clinical Spectrum¶
Symptoms range from mild (heat intolerance, weight loss) to severe (thyrotoxic crisis, myxedema coma). Ophthalmopathy and dermopathy are hallmark features of Graves’ disease.
2. EPIDEMIOLOGY¶
Graves’ disease accounts for 60–80% of thyrotoxicosis cases. Prevalence varies with genetic factors and iodine intake. Women are 3× more likely than men to develop Graves’ disease. Postpartum thyroiditis occurs in 5–10% of women within 6 months of delivery.
2.1 Graves’ Disease¶
Prevalence: 2–3% in women, 0.1–0.5% in men. Risk factors: smoking, iodine intake, genetic predisposition. Peak incidence: 20–50 years.
2.2 Thyroiditis¶
Subacute thyroiditis: 30–50 years, women more affected. Postpartum thyroiditis: 5–10% of women within 6 months post-delivery.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Graves’ disease is an autoimmune disorder driven by TSH receptor antibodies (TSIs) and genetic factors (HLA-DR, CTLA-4). Thyroid-associated ophthalmopathy involves cytokine-mediated fibrosis and IGF-1R signaling. Iodine excess (Jod-Basedow phenomenon) can precipitate hyperthyroidism in susceptible individuals.
Table 396-2: Signs and Symptoms of Thyrotoxicosis¶
| Symptoms | Signs |
|---|---|
| Hyperactivity, irritability, dysphoria | Tachycardia; atrial fibrillation in the elderly |
| Heat intolerance and sweating | Tremor |
| Palpitations | Goiter |
| Fatigue and weakness | Warm, moist skin |
| Weight loss with increased appetite | Muscle weakness, proximal myopathy |
| Diarrhea | Lid retraction or lag |
| Polyuria | Gynecomastia |
3.1 Autoimmune Mechanisms¶
Graves’ disease: TSIs bind TSH-R, stimulating thyroid hormone synthesis. Genetic factors (HLA-DR, CTLA-4) and environmental triggers (smoking, stress) contribute.
3.2 Thyroid-Associated Ophthalmopathy¶
Cytokines (IFN- γ , TNF, IL-1) activate fibroblasts, leading to orbital fibrosis. IGF-1R signaling exacerbates tissue expansion and proptosis.
4. CLINICAL FEATURES¶
Thyrotoxicosis presents with hypermetabolic symptoms (heat intolerance, weight loss) and autonomic features (tachycardia, tremor). Ophthalmopathy (lid retraction, proptosis) and dermopathy (pretibial myxedema) are specific to Graves’ disease.
4.1 Thyrotoxic Crisis¶
Life-threatening exacerbation of hyperthyroidism with fever, delirium, arrhythmias, and multiorgan failure. Mortality: 4–17% despite treatment.
4.2 Ophthalmopathy¶
Graves’ ophthalmopathy: 75% of patients develop signs within 1 year of thyrotoxicosis. Features include lid retraction, proptosis, and diplopia.
5. DIFFERENTIAL DIAGNOSIS¶
Differentiate thyrotoxicosis from subacute thyroiditis (painful goiter, transient thyrotoxic phase), iodine excess (Jod-Basedow phenomenon), and TSH-secreting pituitary tumors.
5.1 Non-Graves Causes¶
Toxic multinodular goiter, toxic adenoma, subacute thyroiditis, and iodine-induced thyrotoxicosis.
5.2 Secondary Hyperthyroidism¶
TSH-secreting pituitary adenoma (normal TSH, elevated free T4) or thyroid hormone resistance syndrome.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnosis requires TSH suppression, elevated free T4, and T3. TRAb testing confirms Graves’ disease. Radioiodine uptake helps distinguish destructive thyroiditis from Graves’ disease.
Table 396-3: Causes of Thyroiditis¶
| Acute | Subacute | Chronic |
|---|---|---|
| Bacterial infection | Viral (granulomatous) thyroiditis | Autoimmunity (Hashimoto’s) |
| Fungal infection | Silent thyroiditis (postpartum) | Riedel’s thyroiditis |
| Radiation thyroiditis | Drug-induced (amiodarone, interferon) | Atrophic thyroiditis |
6.1 Laboratory Tests¶
Measure TSH, free T4, T3, and TRAb. Low TSH with elevated free T4 confirms thyrotoxicosis. Low radioiodine uptake suggests destructive thyroiditis.
6.2 Imaging¶
Radioiodine scan or ultrasound to identify toxic nodules. CT/MRI for orbital involvement in Graves’ ophthalmopathy.
7. MANAGEMENT & TREATMENT¶
Antithyroid drugs (methimazole, propylthiouracil) are first-line. Radioiodine or surgery for refractory cases. Pregnancy requires low-dose LT4 and avoidance of propylthiouracil after 16 weeks.
Table 396-2: Signs and Symptoms of Thyrotoxicosis¶
| Symptoms | Signs |
|---|---|
| Hyperactivity, irritability, dysphoria | Tachycardia; atrial fibrillation in the elderly |
| Heat intolerance and sweating | Tremor |
| Palpitations | Goiter |
| Fatigue and weakness | Warm, moist skin |
| Weight loss with increased appetite | Muscle weakness, proximal myopathy |
| Diarrhea | Lid retraction or lag |
| Polyuria | Gynecomastia |
7.1 Antithyroid Drugs¶
Initial dose: 10–20 mg methimazole or 100–200 mg propylthiouracil every 6–8 h. Titrate based on free T4 levels. Monitor for agranulocytosis.
7.2 Radioiodine and Surgery¶
Radioiodine: 370–555 MBq (10–15 mCi) for euthyroidism. Surgery for large goiters or relapse. Avoid in active ophthalmopathy.
8. PROGNOSIS & COMPLICATIONS¶
Thyrotoxic crisis mortality: 4–17%. Myxedema coma mortality: 20–40%. Long-term complications include ophthalmopathy, osteoporosis, and cardiac arrhythmias. Relapse rates: 30% with glucocorticoids.
8.1 Thyrotoxic Crisis¶
Emergency management: IV glucocorticoids, β -blockers, and antithyroid drugs. Mortality: 4–17% despite treatment.
8.2 Ophthalmopathy¶
Irreversible fibrosis in late stages. Orbital decompression or teprotumumab for severe cases.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Avoid propylthiouracil after 16 weeks; use low-dose LT4. Elderly: Start with 12.5–25 µ g LT4, monitor TSH. Amiodarone-induced thyrotoxicosis requires iodine restriction and antithyroid drugs.
9.1 Pregnancy¶
Target serum free T4 within pregnancy reference range. Avoid iodine excess to prevent fetal goiter.
9.2 Amiodarone Effects¶
High iodine load causes transient suppression of thyroid function. Monitor TSH and free T4 during therapy.
10. KEY POINTS & CLINICAL PEARLS¶
- TRAb testing is critical for diagnosing Graves’ disease in thyrotoxicosis. 2. Thyrotoxic crisis requires immediate glucocorticoids and β -blockers. 3. Ophthalmopathy management includes corticosteroids and orbital decompression. 4. Pregnancy necessitates careful LT4 dose adjustment to avoid fetal hypothyroidism.