Hyperthyroidism

Hyperthyroidism (Thyrotoxicosis)

Definition and Terminology

Hyperthyroidism refers specifically to a condition in which the thyroid gland itself synthesizes and secretes excessive amounts of thyroid hormones, namely triiodothyronine (T3) and thyroxine (T4). Thyrotoxicosis is a broader clinical term describing the syndrome caused by elevated circulating thyroid hormones, regardless of their origin.

The distinction is essential in clinical reasoning:

• Hyperthyroidism → increased hormone production by the thyroid gland
• Thyrotoxicosis → may also result from:
  • exogenous hormone intake
  • thyroid destruction (thyroiditis)
  • ectopic hormone production (e.g., struma ovarii)

Thyroid Hormone Physiology

Thyroid hormone synthesis begins with iodide uptake into follicular cells via the sodium–iodide symporter. This iodide is oxidized and organified by thyroid peroxidase (TPO), leading to iodination of tyrosine residues within thyroglobulin and formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT). These molecules then undergo coupling reactions:

• MIT + DIT → T3
• DIT + DIT → T4

In circulation, T4 is the predominant form, while T3 is the biologically active hormone. Peripheral tissues convert T4 into T3 through deiodinase enzymes (types 1 and 2), whereas type 3 deiodinase produces reverse T3, an inactive form.

At the cellular level, thyroid hormones act via nuclear receptors and increase transcription of genes involved in metabolism, mitochondrial function, and protein synthesis.

Key physiological effects include:

• Increased basal metabolic rate (BMR)
• Increased heat production
• Upregulation of β-adrenergic receptors

Etiology

Hyperthyroidism can be classified based on the underlying mechanism.

Primary hyperthyroidism (TSH-independent) is the most common form and results from intrinsic thyroid pathology:

• Autoimmune:
  • Graves disease (most common cause)

• Nodular disease:
  • Toxic multinodular goiter (Plummer disease)
  • Toxic adenoma (autonomous nodule)

• Iodine-induced:
  • Jod-Basedow phenomenon

Secondary hyperthyroidism (TSH-dependent) is rare and results from excessive stimulation of the thyroid gland:

• TSH-secreting pituitary adenoma
• Rare hypothalamic TRH excess

Thyrotoxicosis without increased synthesis occurs when hormones are released or introduced from outside sources:

• Thyroid destruction:
  • Subacute (de Quervain) thyroiditis
  • Painless/postpartum thyroiditis

• Exogenous sources:
  • Factitious thyrotoxicosis (levothyroxine overdose)

• Other causes:
  • Amiodarone-induced thyrotoxicosis
  • Struma ovarii

Pathophysiology

The central mechanism in hyperthyroidism is an excess of T3 and T4, leading to a generalized increase in metabolic activity across multiple organ systems.

At the cellular level, thyroid hormones increase Na⁺/K⁺-ATPase activity, mitochondrial number, oxygen consumption, and thermogenesis. This results in increased energy expenditure and a negative energy balance, which explains the characteristic weight loss despite increased appetite.

Metabolic effects:

• Carbohydrates:
  • Increased glucose absorption
  • Increased gluconeogenesis and glycogenolysis
  • Increased insulin degradation

→ may lead to glucose intolerance

• Lipids:
  • Increased lipolysis
  • Decreased cholesterol levels

• Proteins:
  • Increased protein catabolism

→ leads to muscle wasting (thyroid myopathy)

Sympathetic interaction:

Thyroid hormones increase β1-adrenergic receptor expression and enhance sensitivity to catecholamines. Symptoms are therefore mainly due to increased responsiveness rather than increased catecholamine levels.

Cardiovascular effects:

• Increased heart rate (positive chronotropy)
• Increased contractility (positive inotropy)
• Increased cardiac output
• Decreased systemic vascular resistance

Long-term consequences include:

• High-output heart failure
• Atrial fibrillation (especially in elderly patients)

Bone metabolism:

• Increased osteoclast activity
• Increased bone turnover

Outcome:

• Net bone loss → osteoporosis

Graves Disease

Graves disease is an autoimmune condition characterized by the production of thyroid-stimulating immunoglobulins (TSI), which bind to and activate the TSH receptor. This results in continuous stimulation of thyroid hormone production and diffuse enlargement of the thyroid gland.

In addition to thyroid involvement, Graves disease presents with extra-thyroidal manifestations:

• Ophthalmopathy (exophthalmos):
  • Inflammation and glycosaminoglycan deposition in orbital tissues
  • Leads to:
    • proptosis
    • diplopia
    • possible optic nerve compression

• Pretibial myxedema:
  • Glycosaminoglycan deposition in the dermis
  • Causes non-pitting edema

6. Clinical Manifestations

The clinical picture reflects a hypermetabolic and hyperadrenergic state.

System	Manifestations
General	Weight loss, heat intolerance, sweating
Cardiovascular	Tachycardia, atrial fibrillation
Neurological	Tremor, anxiety, hyperreflexia
GI	Diarrhea
Musculoskeletal	Muscle weakness, wasting
Skin	Warm, moist skin

Eye signs (Graves):

• Exophthalmos
• Lid lag
• Lid retraction

Thyroid Storm

Thyroid storm is a severe and life-threatening exacerbation of thyrotoxicosis, typically triggered by stressors such as infection, surgery, trauma, or discontinuation of antithyroid medications.

It is characterized by a sudden and extreme increase in thyroid hormone effects and adrenergic activity.

Clinical features:

• Hyperpyrexia
• Severe tachycardia and arrhythmias
• CNS dysfunction (delirium, coma)
• Gastrointestinal symptoms (vomiting, diarrhea)

Management:

• Beta-blockers (e.g., propranolol)
• Antithyroid drugs (PTU initially preferred)
• Iodine (administered after PTU)
• Corticosteroids
• Supportive care

Diagnosis

Diagnosis is based on laboratory and imaging findings.

Laboratory:

• Primary hyperthyroidism:
  • Decreased TSH (most sensitive test)
  • Increased free T4 and/or T3

• Subclinical hyperthyroidism:
  • Decreased TSH
  • Normal T3/T4

Autoimmune markers:

• Thyroid-stimulating immunoglobulins (TSI)
• Anti-thyroid peroxidase (anti-TPO) antibodies

Imaging (RAIU):

• Increased uptake:
  • Graves disease (diffuse)
  • Toxic nodules (focal)

• Decreased uptake:
  • Thyroiditis
  • Exogenous hormone intake

Treatment

Management includes symptomatic control and definitive therapy.

Symptomatic:

• Beta-blockers (e.g., propranolol):
  • Reduce heart rate and tremor
  • Decrease peripheral T4 → T3 conversion (high doses)

Antithyroid drugs:

• Methimazole
• Propylthiouracil (PTU)

Mechanism:

• Inhibit thyroid peroxidase → decreased hormone synthesis
• PTU also inhibits peripheral conversion of T4 to T3

Definitive treatment:

• Radioactive iodine (I-131):
  • Destroys thyroid tissue
  • Often leads to hypothyroidism

• Surgery (thyroidectomy):
  • Indicated in:
    • large goiter
    • suspected malignancy
    • failure of medical therapy

Complications

Untreated or severe hyperthyroidism may lead to:

• Atrial fibrillation
• Heart failure
• Osteoporosis
• Thyroid storm

References

Kumar V, Abbas AK, Aster JC. Robbins and Cotran Pathologic Basis of Disease. 10th ed. Elsevier; 2020.

Hall JE. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021.

Jameson JL, Fauci AS, Kasper DL, Hauser SL, Longo DL, Loscalzo J, eds. Harrison's Principles of Internal Medicine. 21st ed. McGraw-Hill; 2022.

Ross DS, Burch HB, Cooper DS, Greenlee MC, Laurberg P, Maia AL, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421.

National Institute for Health and Care Excellence (NICE). Thyroid disease: assessment and management (NG145). 2019 (updated 2023).