Hyperthyroidism

Etiology and Epidemiology

Graves disease results from activating TSH-receptor antibodies and is a frequent cause of endogenous hormone overproduction. Autonomous functioning nodules or toxic multinodular goiter produce hormone without normal TSH regulation. Destructive thyroiditis releases preformed hormone; excess thyroid replacement causes exogenous thyrotoxicosis. Iodine exposure, amiodarone, pregnancy, and thyroid inflammation complicate attribution. Thyroid disease disproportionately affects women, although clinical presentations span all ages (Esfandiari & McPhee, 2019).

Pathophysiology

T3 binds nuclear receptors, changing gene expression and increasing basal energy expenditure, thermogenesis, oxygen consumption, and tissue responsiveness to catecholamines. Excess hormone increases cardiac rate and contractility, gastrointestinal transit, and bone turnover while driving protein and lipid catabolism. Patients may lose weight despite increased appetite and develop proximal muscle weakness.

Biochemical excess does not reveal the cause by itself. Graves disease commonly has receptor antibodies and diffuse increased synthesis; a toxic adenoma produces focal autonomy; thyroiditis releases stored hormone and often has low uptake; exogenous thyroid hormone suppresses TSH without thyroid overproduction. If radioiodine uptake is inappropriate—for example, during pregnancy or breastfeeding—history, antibody tests, and selected ultrasound findings become more important. T3-predominant thyrotoxicosis can have a disproportionately elevated T3 even when free T4 is not clearly above range.

Clinical Manifestations

Common findings include heat intolerance, sweating, palpitations, tremor, nervousness, insomnia, frequent stools, weight loss, menstrual changes, and tachycardia. Older adults may present mainly with fatigue, weakness, atrial fibrillation, or weight loss rather than obvious adrenergic symptoms. Graves orbitopathy is driven by autoimmune orbital inflammation, not simply by elevated thyroid hormone levels.

Diagnostic Evaluation

Obtain TSH and free T4; assess T3, generally using total T3, when TSH is suppressed but free T4 is not elevated and T3-predominant thyrotoxicosis is suspected. Primary overt hyperthyroidism generally produces suppressed TSH with elevated free T4 and/or T3. Subclinical hyperthyroidism features low TSH with normal circulating thyroid hormones; repeat or clarify transient and medication-related causes when appropriate (Katz, 2026; Penman et al., 2023).

TSH-receptor antibody testing supports Graves disease. Radioiodine uptake and distribution may distinguish increased synthesis (diffuse uptake in Graves; focal or patchy uptake in toxic nodular disease) from destructive thyroiditis or exogenous hormone (low uptake), when clinically appropriate. Uptake scanning is avoided during pregnancy. Ultrasound characterizes anatomy or nodules but cannot independently establish the cause of biochemical thyrotoxicosis (Esfandiari & McPhee, 2019; Penman et al., 2023).

Distinguish persistent subclinical hyperthyroidism from transient TSH suppression by considering repeat testing, age, atrial-arrhythmia risk, osteoporosis risk, medications, and nonthyroidal illness. If TSH is not suppressed despite elevated thyroid hormones, investigate assay interference and uncommon central causes before assuming primary thyroid overproduction.

Etiologic distinction: TSH-receptor antibodies favor Graves disease, while uptake or scintigraphy can distinguish diffuse, focal, patchy, or low uptake where appropriate. Ultrasonography assesses nodules or anatomy but is not a routine substitute for functional tests. Radionuclide uptake is contraindicated in pregnancy and generally avoided during lactation. Biotin supplementation and other assay interferences may create misleading apparent thyroid dysfunction (American Thyroid Association, n.d.).

Management Principles

Beta blockade can reduce adrenergic symptoms when appropriate. Persistent hormone overproduction may be treated with an antithyroid drug, radioiodine, or surgery according to etiology and patient context. Destructive thyroiditis usually calls for symptomatic and cause-directed management rather than antithyroid synthesis blockade. Pregnancy, orbitopathy, arrhythmia, and severe disease alter treatment selection; consult current specialty guidance for medication and monitoring choices.

Mechanism-based treatment: Beta blockers reduce tachycardia and tremor but do not correct excess hormone synthesis. Thionamides inhibit thyroid peroxidase–dependent synthesis and are useful for Graves or toxic nodular hormone overproduction, but not for uncomplicated destructive thyroiditis. Radioiodine damages functioning thyroid tissue, while surgery removes the source. Selection is modified by pregnancy, orbitopathy, goiter characteristics, adverse effects, and patient preferences. Severe sore throat or fever while taking an antithyroid drug requires prompt evaluation for possible agranulocytosis (Katz, 2026; Ross et al., 2016).

Complications and Red Flags

Thyroid storm represents severe, often precipitated systemic decompensation with fever, marked adrenergic cardiovascular stress, CNS dysfunction, and gastrointestinal or hepatic injury. Outside crisis, sustained excess hormone raises the risk of atrial arrhythmias, cardiomyopathy, bone loss, and proximal muscle wasting. The intensity of the syndrome—not the magnitude of one thyroid test—drives emergency assessment.

Pregnancy and preconception alter both diagnosis and risk assessment. In Graves disease, the 2026 ATA guideline favors propylthiouracil over methimazole if an antithyroid drug is required in the first trimester. Patients who are stable on low-dose therapy may be evaluated for careful drug withdrawal with close monitoring instead of assuming therapy must continue. After 16 weeks, the guideline does not make a general recommendation to switch from propylthiouracil to methimazole; the choice depends on clinical circumstances and adverse-effect risks. Radioiodine is contraindicated in pregnancy (Korevaar et al., 2026).

Lactation requires its own treatment assessment: the 2026 ATA guidance generally prefers methimazole over propylthiouracil when antithyroid medication is necessary during breastfeeding because of hepatotoxicity concerns, with individualized infant monitoring as indicated. Therapeutic iodine-131 should not be given during lactation, and breastfeeding must not resume after iodine-131 administration; diagnostic radiopharmaceuticals require isotope-specific precautions (Korevaar et al., 2026).

Clinical Reasoning Priorities

Diagnostic fork: Two patients have suppressed TSH with elevated free T4. One has a diffuse goiter and positive TSH-receptor antibodies, supporting Graves-mediated synthesis; the other has recent painful thyroid inflammation and little or no radionuclide uptake, favoring hormone release. Antithyroid drugs target synthesis and are therefore not interchangeable with symptomatic treatment of destructive thyroiditis. If TSH is not suppressed despite high thyroid hormone, reconsider assay interference, medication exposure, and rare central causes before labeling Graves disease.

High-Yield Distinctions

• Hyperthyroidism is a cause of thyrotoxicosis, not a synonym for every thyrotoxic state.
• A suppressed TSH is usually the expected feedback response to high circulating thyroid hormone in primary disease.
• Graves orbitopathy is an autoimmune orbital disorder, not a direct consequence of hormone concentration.

Related YourDNP Guides

• Back to Pathophysiology
• Arrhythmias

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References

American Thyroid Association. (n.d.). Accuracy and reproducibility. https://www.thyroid.org/professionals/laboratory-services-library/accuracy-and-reproducibility/

Esfandiari, N. H., & McPhee, S. J. (2019). Thyroid disease. In G. D. Hammer & S. J. McPhee (Eds.), Pathophysiology of disease: An introduction to clinical medicine (8th ed., Chapter 20). McGraw-Hill Education.

Katz, M. D. (2026). Thyroid disorders. In M. A. Chisholm-Burns, J. M. Kolesar, P. M. Malone, K. C. Lee, B. P. Bookstaver, & K. R. Matthias (Eds.), Pharmacotherapy principles & practice (7th ed., Chapter 45). McGraw Hill.

Korevaar, T. I. M., Leung, A. M., Alexander, E. K., Bliddal, S., Boelaert, K., Brenta, G., Chou, R., Dhillon-Smith, R., Dosiou, C., Eaton, J. L., Guan, H., Kilpatrick, S. J., Lasserre, B. J., Lee, S. Y., Maraka, S., Meister, K. D., Morris-Wiseman, L. F., Nguyen, C. T., Pearce, E. N., & Shan, Z. (2026). American Thyroid Association 2026 guidelines for thyroid disease in preconception, pregnancy, and postpartum. Thyroid, 36(5), 481–544. https://doi.org/10.1177/10507256261445624

Penman, I. D., Ralston, S. H., Strachan, M. W. J., & Hobson, R. P. (Eds.). (2023). Davidson’s principles and practice of medicine (24th ed.). Elsevier.

Ross, D. S., Burch, H. B., Cooper, D. S., Greenlee, M. C., Laurberg, P., Maia, A. L., Rivkees, S. A., Samuels, M., Sosa, J. A., Stan, M. N., & Walter, M. A. (2016). 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid, 26(10), 1343–1421. https://doi.org/10.1089/thy.2016.0229