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Treatment of Thyroid Disorders - A Comprehensive Overview

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1. HYPOTHYROIDISM

Drug of Choice: Levothyroxine (L-Thyroxine / LT4)

• Typical adult replacement dose: 1.6 µg/kg/day orally
• Start at lower doses (25-50 µg/day) in elderly patients, those with cardiac disease, and long-standing hypothyroidism - then titrate up every 6-8 weeks
• In young, healthy adults, full replacement can be started immediately
• Target: FT4 in the middle-to-upper third of the normal range; TSH 0.5-2.5 mIU/L

• Check TSH at 6-8 weeks after initiating or changing dose
• Once stable, annual TSH monitoring suffices
• TSH should be maintained above 1.0 mIU/L to avoid inadvertent hyperthyroidism

• In secondary (central) hypothyroidism (pituitary failure): dose targets serum T4 at the upper end of normal - TSH cannot be used as the monitoring target
• Always evaluate and replace cortisol first before starting thyroid hormone replacement in secondary hypothyroidism (or combined pituitary failure) - thyroid hormone increases cortisol clearance and can precipitate adrenal crisis
• Thyroid replacement increases clearance of cortisol and may unmask subclinical adrenal insufficiency

• Controversial; not first-line
• May be tried in patients with TSH in therapeutic range who still have persistent hypothyroid symptoms - Textbook of Family Medicine recommends keeping TSH above 1.0 mIU/L if this approach is used

• Treatment is individualized
• Treat if: TSH >10 mIU/L, or TSH <10 mIU/L with positive TPO antibodies, symptoms of hypothyroidism, or atherosclerotic vascular disease
• 3-5% of subclinical cases progress to overt hypothyroidism annually

• Postpartum thyroiditis, subacute thyroiditis - transient hypothyroidism; low-dose LT4 temporarily if symptomatic, then withdrawn

Sources: Textbook of Family Medicine 9e; Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine 22E

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2. HYPERTHYROIDISM - GRAVES' DISEASE

A. Antithyroid Drugs (Thionamides)

• Both inhibit thyroid peroxidase (TPO) - blocking iodine oxidation and organification
• Block coupling of iodotyrosines
• Do NOT block iodide uptake into the gland
• PTU additionally inhibits peripheral deiodination of T4 → T3 (relevant in thyroid storm)
• Both reduce thyroid antibody (TRAb) levels - possibly enhancing spontaneous remission

• Starting dose: 10-20 mg every 12 h (or 20-30 mg once daily)
• Maintenance (titration regimen): 2.5-10 mg/day
• Single 30 mg dose maintains antithyroid effect for >24 hours - once daily dosing is effective
• Preferred over PTU because of longer half-life (6h vs 90 min), better compliance, less hepatotoxicity
• FDA restricts PTU use to: first trimester pregnancy, thyroid storm, and minor adverse reactions to methimazole

• Starting dose: 100-200 mg every 6-8 h
• Loading dose in thyroid storm: 500-1000 mg, then 250 mg every 4 h
• Equivalent: 400 mg PTU = 30 mg methimazole
• Preferred in first trimester pregnancy (more protein-bound, crosses placenta less; methimazole associated with rare congenital malformations)
• Black box warning: risk of severe/fatal hepatotoxicity - avoid in children unless no alternative

• Titration: dose gradually reduced as euthyroidism is restored - preferred (minimizes drug dose)
• Block-replace: high-dose antithyroid drug + LT4 supplementation to prevent iatrogenic hypothyroidism

• T4 and T3 normalize in 4-6 weeks
• TSH may remain suppressed for weeks even after T4/T3 normalize - do not use TSH to monitor early response
• Remission rates: 30-60% after 12-18 months of therapy
• Higher remission if TRAb levels normalize
• If TRAb persist after 12-18 months, proceed to radioiodine or surgery

• Agranulocytosis warning: Do NOT monitor neutrophil counts routinely (onset too rapid). Instruct patients to stop drug immediately and seek care for fever, sore throat, or infection symptoms. If agranulocytosis occurs, neither drug can ever be given again.

B. Radioactive Iodine (RAI / ¹³¹I) Therapy

• Mechanism: Kills thyroid follicular cells, reducing or eliminating hormone production
• Goal: Ablate the gland and render patient hypothyroid (then treat with LT4)
• Efficacy: Ameliorates hyperthyroidism in ~90% of Graves' patients within 6 months
• Effective for: Graves' disease, autonomous hyperfunctioning nodules, toxic multinodular goiter
• May worsen Graves' ophthalmopathy (exacerbation risk; prophylactic glucocorticoids may be used)
• Contraindicated in pregnancy and breastfeeding
• In Graves' disease: antithyroid drug is often given first to deplete hormone stores, then discontinued before RAI

• Give antithyroid drug first in severe thyrotoxicosis to reduce hormone stores
• Stop PTU/methimazole ~5-7 days before RAI (methimazole may reduce RAI efficacy)
• After RAI, lifelong LT4 replacement is usually required

C. Thyroidectomy

• Indicated when: large goiter causing compressive symptoms, suspected/confirmed malignancy, patient preference, pregnancy (second trimester if medical therapy fails), severe ophthalmopathy
• Pre-operative preparation: Render patient euthyroid with antithyroid drugs first; add beta-blockers and Lugol's solution to reduce vascularity
• Total thyroidectomy is standard for Graves' disease
• Post-operative care: Lifelong LT4 replacement; monitor for hypoparathyroidism (hypocalcemia) and recurrent laryngeal nerve injury

D. Adjunctive Agents

• Beta-blockers (propranolol, atenolol, metoprolol): Control adrenergic symptoms (tachycardia, tremor, sweating, anxiety) rapidly. Do not correct thyrotoxicosis itself. Used as bridge while awaiting thionamide effect.
• Potassium iodide (SSKI) / Lugol's solution: Supraphysiologic iodine blocks thyroid hormone release (Wolff-Chaikoff effect). Used in severe thyrotoxicosis, pre-surgery, or in patients intolerant to antithyroid drugs. Must be given only after antithyroid drug has been started (to prevent iodine from stimulating new hormone synthesis).
• Lithium carbonate: Blocks thyroid hormone release; used short-term in patients intolerant to antithyroid drugs and iodine.
• Cholestyramine: Enhances hepatic metabolism of T4; used adjunctively in severe cases.

Sources: Harrison's Principles of Internal Medicine 22E; Goldman-Cecil Medicine; Katzung's Basic and Clinical Pharmacology 16E

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3. THYROID STORM (Thyrotoxic Crisis)

Treatment Steps (in Order):

• Oxygen, IV access, cardiac monitoring
• Fever: External cooling (ice packs, cooling blankets); acetaminophen 325-650 mg PO/PR every 4-6 h (aspirin is contraindicated - displaces thyroid hormone from binding proteins, increases free T4/T3)
• IV fluids (isotonic saline with 5% dextrose if blood glucose is low - high metabolic rate depletes glycogen)
• Nutrition: glucose, multivitamins, thiamine, folate

• Propranolol 0.5-1 mg IV over 10 min, then 1-2 mg IV every few hours - OR - 60-80 mg PO every 4 h
• Esmolol 250-500 µg/kg IV loading dose, then 50-100 µg/kg/min (for titration)
• In reactive airway disease: cardioselective agents (atenolol, metoprolol) or calcium channel blockers

• Methimazole 20 mg every 6 h PO
• OR PTU loading dose 500-1000 mg PO, then 250 mg every 4 h (PTU preferred if rapid T4→T3 block needed; also preferred in first trimester)

• Lugol's solution 8-10 drops PO every 6-8 h
• OR SSKI 5 drops (250 mg) PO every 6 h
• OR Lithium carbonate 300 mg PO every 6 h if iodine allergic or agranulocytosis with thionamides (maintain serum lithium ~1 mEq/L; contraindicated in pregnancy)

• PTU (as above) - its deiodination-blocking effect is most relevant here
• Glucocorticoids: Hydrocortisone 100 mg IV every 8 h (also treats possible relative adrenal insufficiency from hypermetabolism, and blocks T4→T3 conversion)
• Iodinated contrast agents (iopanoic acid, sodium ipodate): also block conversion

• Infection, trauma, surgery, iodine load, medication non-compliance, etc.

Source: Tintinalli's Emergency Medicine - A Comprehensive Study

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4. MYXEDEMA COMA (Severe Hypothyroid Crisis)

• IV thyroid hormone replacement is mandatory (poor GI absorption in severe hypothyroidism)
• LT4 IV: 200-400 µg loading dose, then 1.6 µg/kg/day IV
• Some protocols add T3 (liothyronine) IV in the critically ill since T4-to-T3 conversion is impaired
• Hydrocortisone 50-100 mg IV every 6-8 h: given empirically before thyroid hormone (until adrenal insufficiency excluded)
• Supportive: warming blankets, ventilatory support, IV fluids, glucose

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5. THYROIDITIS - SUBTYPE-SPECIFIC TREATMENT

Subacute (De Quervain's) Thyroiditis:

• Aspirin 600 mg every 4-6 h (high-dose) - first line for mild-moderate symptoms
• NSAIDs as alternative
• Add a PPI/gastroprotective agent
• Prednisone 15-40 mg/day (taper over 6-8 weeks) for severe local or systemic symptoms, or if NSAIDs fail
• Beta-blockers (propranolol) for thyrotoxic symptoms - antithyroid drugs have no role (hormone release, not overproduction)
• LT4 50-100 µg/day if hypothyroid phase is prolonged
• Monitor TSH and FT4 every 2-4 weeks

Silent / Postpartum Thyroiditis:

• Glucocorticoids NOT indicated
• Propranolol 20-40 mg 3-4 times daily for severe thyrotoxic symptoms (brief course)
• LT4 if hypothyroid phase; withdraw after 6-9 months (recovery expected)
• Annual follow-up (risk of permanent hypothyroidism)
• Condition may recur in subsequent pregnancies

Hashimoto's (Chronic Autoimmune) Thyroiditis:

• Once hypothyroidism develops: LT4 1.6 µg/kg/day - titrate to normal TSH
• Annual TSH monitoring
• No treatment to reverse the autoimmune process

Drug-Induced Thyroiditis (IFN-α, checkpoint inhibitors, tyrosine kinase inhibitors):

• Manage same as silent thyroiditis
• Routine thyroid function monitoring during these therapies is recommended (ASCO guidelines for checkpoint inhibitors)

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6. THYROID CANCER

Differentiated Thyroid Cancer (Papillary and Follicular):

• Total thyroidectomy is the primary treatment
• Neck dissection if lymph node involvement

• Post-surgical RAI to ablate residual thyroid tissue and metastases
• Indications determined by risk stratification (tumor size, extent, histology)
• Evaluation for recurrence: withdraw LT4 for 4-6 weeks (deliberate hypothyroidism) or use recombinant TSH (rhTSH) to stimulate RAI uptake - monitoring serum thyroglobulin and whole-body scan

• Lifelong LT4 at doses that suppress TSH below normal (0.1-0.5 mIU/L for intermediate risk; <0.1 for high-risk)
• Goal: prevent TSH-stimulated tumor growth
• Degree of suppression is risk-adapted - relaxed in low-risk patients after remission

• Sorafenib, lenvatinib - multikinase inhibitors for radioiodine-refractory differentiated thyroid cancer
• BRAF inhibitors (vemurafenib, dabrafenib) for BRAF V600E-mutated papillary thyroid cancer
• RET inhibitors (selpercatinib, pralsetinib) for RET fusion-positive thyroid cancer and medullary thyroid cancer with RET mutations

Medullary Thyroid Cancer (MTC):

• Surgery (total thyroidectomy + central neck dissection) is mainstay
• RET mutation testing mandatory (hereditary MTC in MEN2)
• Vandetanib or cabozantinib for progressive/metastatic MTC
• RAI is not effective (parafollicular C-cell origin, does not concentrate iodine)

Anaplastic Thyroid Cancer:

• Extremely aggressive; surgery if resectable
• Combination chemoradiation (doxorubicin-based)
• BRAF V600E mutation present in ~50%: dabrafenib + trametinib shows significant benefit in BRAF-mutated anaplastic cancer

Sources: Katzung's Basic and Clinical Pharmacology 16E; Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine 22E; 2025 ATA Guidelines (PMID: 40844370)

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7. THYROID DISEASE IN SPECIAL POPULATIONS

Pregnancy:

• Hypothyroidism: LT4 requirements increase ~30-50% - check TSH at 4-6 weeks after diagnosis, then each trimester; titrate to TSH <2.5 mIU/L in first trimester
• Hyperthyroidism: PTU is preferred in first trimester (methimazole teratogenic - methimazole embryopathy: choanal atresia, aplasia cutis); switch to methimazole in second/third trimester (PTU hepatotoxicity risk)
• Both thionamides cross the placenta; maintain FT4 at or above upper limit of normal to avoid fetal hypothyroidism
• RAI is absolutely contraindicated in pregnancy

Elderly:

• Start LT4 at low doses (25 µg/day); titrate slowly due to risk of precipitating angina or arrhythmia
• Subclinical hyperthyroidism (low TSH) is associated with 2-3x higher risk of atrial fibrillation

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Summary Table: Key Drugs

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• Harrison's Principles of Internal Medicine 22E (2025) - Graves' disease, thyroiditis treatment
• Goldman-Cecil Medicine - Antithyroid drug therapy, radioiodine
• Katzung's Basic and Clinical Pharmacology 16E - Pharmacokinetics and toxicity
• Tintinalli's Emergency Medicine - Thyroid storm management
• Textbook of Family Medicine 9e - Hypothyroidism, subclinical thyroid disease
• 2025 ATA Management Guidelines for Differentiated Thyroid Cancer (PMID: 40844370)