Does hypothyroidism (underactive thyroid) cause hypokalemia (low potassium levels)?

Hypothyroidism Does NOT Cause Hypokalemia—It Causes Hyperkalemia

Hypothyroidism is associated with hyperkalemia (elevated potassium), not hypokalemia (low potassium). The premise of your question contains a fundamental error that needs correction based on current evidence.

The Actual Relationship: Hypothyroidism and Hyperkalemia

Mechanism of Potassium Elevation in Hypothyroidism

• Acute hypothyroidism causes significant increases in serum potassium levels, primarily through decreased estimated glomerular filtration rate (eGFR) and decreased plasma renin activity 1
• The reduction in renal function and suppressed renin-angiotensin-aldosterone system activity impairs potassium excretion 1
• Hypothyroidism reduces cardiac output, which contributes to decreased renal perfusion and subsequent potassium retention 2
• Thyroid hormone normally stimulates Na-K-ATPase activity; its deficiency in hypothyroidism impairs cellular potassium uptake, leading to extracellular accumulation 1

Clinical Evidence

• A prospective study of 46 patients undergoing levothyroxine withdrawal demonstrated mean serum potassium levels significantly increased at 4 weeks after developing hypothyroidism 1
• The same study showed serum potassium values correlated significantly with both eGFR and plasma renin activity in the hypothyroid state 1
• Hyperkalemia was more common in patients with elevated TSH (7%) compared to those with normal TSH (4%), while hypokalaemia was actually less common 3

When Hypokalemia Occurs With Thyroid Disease

Hyperthyroidism, Not Hypothyroidism

• Hyperthyroidism (the opposite condition) can be associated with hypokalemia through thyrotoxic periodic paralysis, where potassium shifts intracellularly 4
• Hyperthyroidism increases Na-K-ATPase activity, driving potassium into cells 4

Concurrent Conditions

• If a patient with hypothyroidism presents with hypokalemia, investigate alternative causes rather than attributing it to the thyroid disorder 4
• Primary aldosteronism can coexist with thyroid disease and causes hypokalemia with hypertension 5, 4
• Diuretic use, particularly thiazide and loop diuretics, commonly causes hypokalemia in patients who may coincidentally have hypothyroidism 5
• Gastrointestinal losses (vomiting, diarrhea) and renal losses are common causes to evaluate 6

Associated Electrolyte Disturbances in Hypothyroidism

Hyponatremia: The Classic Association

• Hyponatremia is the characteristic electrolyte abnormality in hypothyroidism, occurring in 14% of patients with elevated TSH compared to 9% with normal TSH 3
• The mechanism involves elevated antidiuretic hormone levels secondary to decreased cardiac output, impairing free water excretion 7
• Severe hypothyroidism and myxedema are particularly associated with reduced sodium levels below 135 mmol/L 7

Other Electrolyte Changes

• Serum phosphate levels are higher in hypothyroid patients, with significant correlation between TSH and phosphate 3
• Calcium and magnesium also correlate with TSH levels 3
• Hypomagnesemia may be an associated finding in some cases of hypothyroidism and can contribute to hypocalcemia 2

Clinical Pitfalls to Avoid

• Do not assume hypokalemia is caused by hypothyroidism—this represents a fundamental misunderstanding of thyroid-electrolyte relationships 1, 3
• When hypothyroid patients present with hypokalemia, mandatory evaluation includes: aldosterone-to-renin ratio for primary aldosteronism 5, medication review for diuretics or other potassium-wasting drugs 5, and assessment for gastrointestinal or renal losses 6
• In patients with both hyperthyroidism and hypokalemia presenting with renal potassium wasting and metabolic alkalosis, consider concurrent primary aldosteronism rather than attributing everything to thyrotoxic periodic paralysis 4
• The association between thyroid dysfunction and electrolyte disorders is clinically relevant primarily in marked hypothyroidism or hyperthyroidism, not mild cases 3

Monitoring Recommendations

• TSH determination is mandatory during evaluation of patients with any electrolyte abnormality, particularly hyponatremia 7
• For hypothyroid patients, monitor sodium levels as the primary electrolyte concern 3, 7
• Potassium monitoring is important when initiating thyroid hormone replacement in hypothyroid patients, as correction may normalize the hyperkalemia 1
• Thyroid function should be assessed annually in patients with known thyroid disease to prevent electrolyte complications 2