Can hypothyroidism cause seizures, and what is the appropriate workup and management for a patient with hypothyroidism who presents with convulsions?

Can Hypothyroidism Cause Seizures?

Yes, severe hypothyroidism can cause seizures, though this is rare and typically occurs only in the context of myxedema coma or severe metabolic derangements—not in routine subclinical or mild overt hypothyroidism.

Mechanism and Clinical Context

Hypothyroidism itself does not directly lower the seizure threshold in the way that hypocalcemia or hyponatremia do. However, myxedema coma—the life-threatening extreme of hypothyroidism—can precipitate seizures through secondary metabolic disturbances 1, 2. The seizures are not caused by low thyroid hormone per se, but rather by:

• Severe hyponatremia, which is common in myxedema coma and directly increases neuronal excitability 1
• Hypoxemia and hypercarbia from hypoventilation, which lower seizure threshold 1, 2
• Cerebral hypoperfusion from bradycardia, hypotension, and hypothermia 3, 4

Seizures as a presenting feature of myxedema coma carry a very high mortality rate 1, 2. One case series documented status epilepticus as the initial manifestation, with the patient dying despite treatment 2.

Diagnostic Workup for Seizures in a Patient with Known Hypothyroidism

When a patient with hypothyroidism presents with new-onset seizures, follow this algorithmic approach:

Immediate Assessment (Emergency Department)

• Check serum sodium immediately—hyponatremia is the most common metabolic cause of seizures in hypothyroid patients 1
• Measure ionized calcium and magnesium—hypocalcemia can cause seizures and may coexist with hypothyroidism, particularly in patients with autoimmune polyglandular syndromes 5
• Obtain TSH and free T4 urgently to assess severity of hypothyroidism 1, 4
• Check glucose, creatinine, and arterial blood gas to identify hypoglycemia, uremia, or respiratory failure 1, 2
• Assess core body temperature—hypothermia (<35°C) suggests myxedema coma 4, 2
• Obtain 12-lead ECG to evaluate for bradycardia and prolonged QT interval 6

Clinical Features Suggesting Myxedema Coma

• Altered mental status ranging from confusion to coma (despite the name, frank coma is not required) 4
• Hypothermia (core temperature <35°C) 1, 2
• Bradycardia and hypotension 3, 4
• Hypoventilation with hypoxemia and hypercarbia 1, 2
• Precipitating event: infection, surgery, cold exposure, medication non-compliance, or malnutrition 4

Seizure Management

For active seizures or status epilepticus:

• Administer benzodiazepine immediately—lorazepam 0.1 mg/kg IV or diazepam 0.15 mg/kg IV 6
• Ensure airway patency and adequate oxygenation; prepare for intubation if respiratory compromise worsens 6
• If seizure continues >5 minutes after first benzodiazepine, administer second-line agents (levetiracetam or fosphenytoin) 6

Correct underlying metabolic abnormalities:

• If sodium <120 mEq/L, administer hypertonic saline cautiously (target correction 4-6 mEq/L in first 24 hours to avoid osmotic demyelination) 1
• If ionized calcium is low, give IV calcium gluconate 50-100 mg/kg slowly over 10 minutes with continuous ECG monitoring 5
• Check and correct magnesium deficiency immediately—calcium replacement will fail if magnesium is not corrected first 5

Thyroid Hormone Replacement in Myxedema Coma

If myxedema coma is confirmed (TSH typically >100 mIU/L, free T4 undetectable):

• Administer IV levothyroxine 200-400 mcg loading dose, followed by 50-100 mcg daily 4
• Add IV liothyronine (T3) 5-20 mcg every 8-12 hours in critically ill patients 4
• Rule out adrenal insufficiency before starting thyroid hormone—give hydrocortisone 100 mg IV every 8 hours empirically until cortisol results return 7

Critical pitfall: Starting thyroid hormone before corticosteroids can precipitate adrenal crisis in patients with concurrent adrenal insufficiency 7.

When Seizures Are NOT Caused by Hypothyroidism

Do not attribute seizures to hypothyroidism if:

• TSH is only mildly elevated (e.g., TSH 10-50 mIU/L with normal free T4)—this represents subclinical hypothyroidism, which does not cause seizures 7
• Patient is clinically stable without hypothermia, altered mental status, or severe metabolic derangements 1, 4
• Sodium, calcium, and glucose are normal—look for other causes of seizures (structural brain lesion, primary epilepsy, infection) 8, 6

Important Caveat: Transient TSH Elevation After Seizures

TSH can be transiently elevated following seizures (including non-convulsive status epilepticus), mimicking subclinical hypothyroidism 9. One case report documented TSH elevation to 8.7 mIU/L immediately after non-convulsive status epilepticus, which normalized spontaneously within weeks 9. Do not start levothyroxine based on a single elevated TSH obtained shortly after a seizure—repeat TSH and free T4 in 3-6 weeks to confirm persistence 7, 9.

Misdiagnosing post-seizure TSH elevation as hypothyroidism and treating with levothyroxine can paradoxically provoke further seizures 9.

Long-Term Management After Seizure Resolution

If seizures were provoked by myxedema coma or severe hyponatremia:

• Long-term antiepileptic drugs are NOT indicated once the underlying metabolic cause is corrected 6
• Optimize levothyroxine dosing to maintain TSH 0.5-4.5 mIU/L and normal free T4 7
• Monitor TSH every 6-8 weeks until stable, then every 6-12 months 7
• Ensure medication compliance—non-compliance is the most common cause of recurrent myxedema coma 7

If seizures occurred in the setting of hypocalcemia:

• Hypocalcemic seizures generally resolve with calcium supplementation alone; anticonvulsants may be discontinued once calcium normalizes 5
• Check parathyroid hormone (PTH) levels to identify hypoparathyroidism 5
• Screen for autoimmune polyglandular syndrome if both hypothyroidism and hypoparathyroidism are present 5

Summary Algorithm

1. New seizure in hypothyroid patient → Check sodium, calcium, magnesium, TSH, free T4, glucose, temperature
2. If TSH >100 mIU/L + hypothermia + altered mental status → Myxedema coma → IV levothyroxine + hydrocortisone + correct electrolytes
3. If sodium <120 mEq/L → Hyponatremia-induced seizure → Hypertonic saline + treat hypothyroidism
4. If ionized calcium low → Hypocalcemia-induced seizure → IV calcium gluconate + check PTH
5. If TSH mildly elevated (10-50 mIU/L) with normal sodium/calcium → Seizure NOT due to hypothyroidism → Investigate other causes
6. If TSH elevated immediately post-seizure → Repeat in 3-6 weeks before treating (may be transient) 9

Bottom line: Hypothyroidism causes seizures only in the context of myxedema coma or severe secondary metabolic derangements (hyponatremia, hypocalcemia). Routine subclinical or mild overt hypothyroidism does not cause seizures. Always correct electrolytes and rule out adrenal insufficiency before attributing seizures to thyroid dysfunction.