Residency · Residency · Internal Medicine

Thyroid Disorders: From Subclinical Disease to Thyroid Storm

Overview

Thyroid disorders are among the most common endocrine conditions encountered in internal medicine. TSH is the primary screening test and is highly sensitive for primary thyroid dysfunction. The clinical spectrum ranges from subclinical disease (biochemical abnormality without symptoms) to life-threatening emergencies (thyroid storm, myxedema coma). Management decisions must weigh treatment benefits against risks, particularly in elderly and subclinical populations.

Thyroid Physiology

The hypothalamic-pituitary-thyroid axis operates through TRH from the hypothalamus stimulating TSH release from the anterior pituitary, which in turn stimulates the thyroid gland to produce T4 (the prohormone) and T3 (the active hormone). Negative feedback from T3 and T4 suppresses TSH and TRH production. Approximately 80% of circulating T3 is produced by peripheral conversion of T4 through deiodinase enzymes. Free T4 and free T3 are the biologically active fractions unbound to protein.

Hypothyroidism

Overt Hypothyroidism

Overt hypothyroidism presents with elevated TSH and low free T4. Primary hypothyroidism accounts for 95% of cases, with Hashimoto thyroiditis being the most common cause in iodine-sufficient areas, followed by post-thyroidectomy states, post-radioiodine therapy, iodine deficiency, and medications (amiodarone, lithium, checkpoint inhibitors). Central hypothyroidism is rare and results from pituitary or hypothalamic disease; TSH may be low, normal, or mildly elevated with low free T4. Symptoms include fatigue, cold intolerance, weight gain, constipation, dry skin, bradycardia, delayed deep tendon reflexes, and periorbital edema.

Subclinical Hypothyroidism

Subclinical hypothyroidism is defined by elevated TSH (4.5-10 mIU/L) with normal free T4, with a prevalence of 5-10% of adults that increases with age. Treatment is generally recommended when TSH exceeds 10 mIU/L. For TSH between 4.5 and 10, the decision is more nuanced: treatment may be favored for symptomatic patients, those with positive TPO antibodies, those who are pregnant or planning pregnancy, and patients under 65-70 years. Treatment may be deferred in elderly patients over 70, as the TRUST trial showed no benefit of levothyroxine in adults 65 or older with subclinical hypothyroidism. TSH should be rechecked in 6-12 weeks to confirm persistence before starting therapy.

Treatment of Hypothyroidism

Levothyroxine (T4) is the standard of care. The full replacement dose is approximately 1.6 mcg/kg/day. Full-dose initiation is appropriate in young, healthy patients, while elderly patients and those with coronary artery disease should start low (25-50 mcg/day) to avoid angina or arrhythmia. The medication should be taken on an empty stomach, 30-60 minutes before breakfast (or at bedtime, 3 or more hours after the last meal), and separated from calcium, iron, and PPIs by 4 hours. TSH is rechecked in 6-8 weeks after initiation or dose change, targeting 0.5-2.5 for most patients. Combination T4/T3 therapy is not routinely recommended as trials have not shown convincing superiority over T4 alone, though some patients report subjective benefit.

<image>Diagnostic algorithm for thyroid function testing showing TSH-based approach with pathways for primary hypothyroidism, subclinical hypothyroidism, hyperthyroidism, and central thyroid disease</image>

Hyperthyroidism

Overt Hyperthyroidism

Overt hyperthyroidism presents with suppressed TSH (below 0.1 mIU/L) and elevated free T4 and/or free T3. Causes include Graves disease (the most common, featuring diffuse goiter, ophthalmopathy, pretibial myxedema, and positive TSI/TRAb), toxic multinodular goiter, toxic adenoma (single hot nodule), thyroiditis (subacute/de Quervain, postpartum, drug-induced, all transient due to release of preformed hormone), exogenous thyroid hormone (factitious or iatrogenic), and amiodarone-induced thyrotoxicosis (Type 1: iodine-induced in a predisposed gland; Type 2: destructive thyroiditis). Symptoms include weight loss, heat intolerance, tremor, palpitations, anxiety, diarrhea, menstrual irregularity, and lid lag or retraction.

Differentiating Hyperthyroidism Causes

The radioactive iodine uptake (RAIU) scan shows diffusely increased uptake in Graves disease, focal increase in toxic adenoma or toxic multinodular goiter, and low or absent uptake in thyroiditis, exogenous hormone use, or iodine excess. TSI (thyroid-stimulating immunoglobulin) or TRAb positivity confirms Graves disease and may allow avoidance of RAIU scanning when the clinical picture is clear.

Treatment Options for Graves Disease

Antithyroid drugs include methimazole (preferred for once-daily dosing and fewer side effects) and PTU (preferred in first trimester pregnancy and thyroid storm because it blocks peripheral T4-to-T3 conversion). Duration is typically 12-18 months with approximately 50% remission rate. Side effects include agranulocytosis (0.2-0.5%; WBC should be checked if fever or sore throat develops), hepatotoxicity, and rash. Radioactive iodine (RAI) provides definitive treatment that typically results in hypothyroidism; it is contraindicated in pregnancy, breastfeeding, and moderate-severe Graves ophthalmopathy (which it can worsen). Thyroidectomy is indicated for large goiters with compressive symptoms, suspected malignancy, contraindications to other treatments, or severe ophthalmopathy, with risks including hypoparathyroidism and recurrent laryngeal nerve injury.

Subclinical Hyperthyroidism

Subclinical hyperthyroidism features suppressed TSH with normal free T4 and T3. It carries risks of progression to overt hyperthyroidism, atrial fibrillation (especially if TSH is below 0.1), and osteoporosis. Treatment is indicated when TSH is persistently below 0.1, in patients over 65, those with cardiac risk factors or osteoporosis, or symptomatic patients.

<image>Radioactive iodine uptake patterns distinguishing Graves disease (diffuse uptake), toxic multinodular goiter (patchy uptake), toxic adenoma (focal uptake), and thyroiditis (low/absent uptake)</image>

Thyroid Storm

Definition

Thyroid storm is a life-threatening exaggeration of thyrotoxicosis with multiorgan dysfunction, carrying mortality of 10-30% even with treatment. The Burch-Wartofsky Point Scale (BWPS) uses temperature, CNS effects, GI-hepatic dysfunction, heart rate, heart failure, and precipitant history to score severity: a score of 45 or higher is highly suggestive of thyroid storm, 25-44 suggests impending storm, and below 25 makes storm unlikely.

Precipitants

Common precipitants include infection (the most common), surgery, trauma, DKA, iodine load (contrast, amiodarone), medication non-compliance, and pregnancy or labor.

Clinical Features

Thyroid storm presents with high fever (above 104 degrees F / 40 degrees C), severe tachycardia or atrial fibrillation, altered mental status (agitation, delirium, coma), GI symptoms (nausea, vomiting, diarrhea, jaundice with hepatic dysfunction being ominous), and heart failure or cardiovascular collapse.

Management (All therapies initiated simultaneously)

PTU is preferred over methimazole because it blocks peripheral T4-to-T3 conversion; dosing is 500-1000 mg loading dose then 250 mg every 4 hours. Iodine (Lugol's or SSKI) is given 1 hour after PTU to avoid using iodine as substrate and blocks thyroid hormone release. A beta-blocker (propranolol 60-80 mg orally every 4 hours, which also inhibits T4-to-T3 conversion, or esmolol IV if needed) controls adrenergic symptoms. Glucocorticoids (hydrocortisone 100 mg IV every 8 hours) block T4-to-T3 conversion and treat potential relative adrenal insufficiency. Supportive care includes cooling measures, IV fluids, and acetaminophen (aspirin is avoided because it displaces T4 from binding proteins), with ICU admission. Cholestyramine 4 g orally four times daily binds thyroid hormone in the GI tract as adjunctive therapy. Plasmapheresis or plasma exchange is reserved for refractory cases.

Myxedema Coma

Definition

Myxedema coma is a life-threatening decompensation of severe hypothyroidism with mortality of 30-60%, typically occurring in elderly patients with longstanding untreated or undertreated hypothyroidism.

Precipitants

Common precipitants include infection, cold exposure, sedative medications (opioids, benzodiazepines), stroke, surgery, and trauma.

Clinical Features

The cardinal feature is hypothermia, accompanied by altered mental status progressing to coma, bradycardia, hypotension, hypoventilation (with CO2 retention), hyponatremia (from impaired free water excretion), delayed deep tendon reflexes, non-pitting edema, and effusions (pericardial, pleural).

Management

IV levothyroxine is administered as a loading dose of 200-400 mcg followed by 50-100 mcg IV daily. IV liothyronine (T3) at 5-20 mcg then 2.5-10 mcg IV every 8 hours provides faster-acting hormone, though its use is controversial and some experts use T4 alone. Stress-dose hydrocortisone (100 mg IV every 8 hours) is given to rule out concurrent adrenal insufficiency before giving T4 alone, as T4 can precipitate adrenal crisis. Supportive care includes passive rewarming (active rewarming can cause vasodilation and shock), mechanical ventilation if hypoventilation is present, and vasopressors for refractory hypotension. Sedatives must be avoided due to prolonged metabolism in hypothyroidism.

<image>Emergency management of thyroid storm and myxedema coma showing parallel treatment algorithms with medications, doses, and sequencing of therapies</image>

Thyroid Nodules (Brief Overview)

Thyroid nodules are palpable in 5% of adults and incidental on imaging in up to 50%. Workup begins with TSH: if suppressed, a RAIU scan is performed (hot nodules rarely being malignant). If TSH is normal or elevated, ultrasound characterization guides fine needle aspiration biopsy when size and appearance criteria are met (using TI-RADS or ATA guidelines). The Bethesda classification guides management after FNA.

Clinical Pearls

In thyroid storm, PTU must be given before iodine because iodine given first can fuel hormone synthesis in an already overactive gland (the Jod-Basedow effect). Propranolol is the preferred beta-blocker in thyrotoxicosis because it additionally blocks peripheral T4-to-T3 conversion. Myxedema coma should be treated empirically if clinically suspected without waiting for laboratory confirmation. Amiodarone-induced thyrotoxicosis can be challenging: Type 1 responds to thionamides and Type 2 to steroids, but in practice both may be started empirically as the distinction is often difficult. Subclinical hypothyroidism in the elderly (especially TSH below 10) often does not require treatment and may even be adaptive, as the TRUST trial showed no quality-of-life benefit. Biotin supplements can interfere with thyroid function assays, producing falsely low TSH and falsely high free T4 in competitive assays; biotin should be held for 48 hours before thyroid labs.

References

• Ross DS, et al. 2016 ATA Guidelines for Diagnosis and Management of Hyperthyroidism. Thyroid. 2016;26:1343-1421.
• Garber JR, et al. ATA/AACE Clinical Practice Guidelines for Hypothyroidism in Adults. Thyroid. 2012;22:1200-1235.
• Stott DJ, et al. Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism (TRUST). N Engl J Med. 2017;376:2534-2544.
• Akamizu T, et al. Diagnostic Criteria, Clinical Features, and Incidence of Thyroid Storm. Thyroid. 2012;22:661-679.
• Burch HB, Wartofsky L. Life-Threatening Thyrotoxicosis: Thyroid Storm. Endocrinol Metab Clin North Am. 1993;22:263-277.

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