Can Synthroid (levothyroxine) cause urinary symptoms in patients with hypothyroidism?

Synthroid Does Not Directly Cause Urinary Symptoms

Synthroid (levothyroxine) itself does not cause urinary symptoms in patients with hypothyroidism. However, there is one rare clinical scenario where urinary findings may be indirectly related to thyroid hormone therapy.

The Rare Exception: Nephrotic Syndrome

• In patients who develop nephrotic syndrome (massive proteinuria), levothyroxine requirements may increase dramatically because thyroid hormone is lost through the kidneys 1
• This presents as rising TSH levels despite previously adequate levothyroxine dosing, and dipstick urine testing reveals significant proteinuria 1
• The urinary symptoms in this case are from the underlying kidney disease (nephrotic syndrome), not from the levothyroxine itself 1

Why This Matters Clinically

• If a patient on stable levothyroxine therapy develops rising TSH levels despite good compliance, perform a urinalysis to check for proteinuria before simply increasing the dose 1
• Nephrotic syndrome causes urinary loss of thyroxine-binding proteins, leading to increased thyroid hormone clearance and higher dose requirements 2
• This is an extremely rare presentation but is easily detected with simple dipstick urine testing 1

Common Pitfalls to Avoid

• Do not attribute non-specific symptoms to levothyroxine without evidence of over- or under-treatment 3
• Approximately 25% of patients on levothyroxine are unintentionally maintained on doses that suppress TSH completely, which increases risks for atrial fibrillation and osteoporosis—but not urinary symptoms 4, 5
• The adverse effects of levothyroxine are signs of thyrotoxicosis in overdose (tachycardia, tremor, sweating), not urinary complaints 3

Bottom Line

Levothyroxine does not cause urinary symptoms. If urinary symptoms develop in a patient taking Synthroid, investigate other causes. The only connection between levothyroxine and urine is the rare scenario of nephrotic syndrome causing increased thyroid hormone loss in urine, which manifests as rising TSH—not as patient-reported urinary symptoms 1.