Levothyroxine Can Cause Fever
Yes, levothyroxine can cause fever, but this occurs specifically in the context of overdose or excessive dosing that leads to hyperthyroidism/thyrotoxicosis. 1
Mechanism and Clinical Context
Fever is listed as an adverse reaction in the FDA drug label for levothyroxine, occurring as part of the hyperthyroid state induced by therapeutic overdosage 1. The fever results from:
- Increased metabolic rate from excess thyroid hormone
- Heat intolerance and excessive sweating accompanying the hypermetabolic state
- Thyroid storm in severe cases of overdose
When Fever Occurs
Overdose Scenarios
In documented cases of levothyroxine overdose, fever (>38°C) appears as part of the clinical presentation of thyrotoxicosis 2. A case report demonstrated a patient who ingested 12 mg levothyroxine developed fever of 38°C on day 3 post-ingestion, meeting criteria for thyroid storm 2.
Chronic Overtreatment
The FDA label explicitly warns that adverse reactions from levothyroxine are "primarily those of hyperthyroidism due to therapeutic overdosage," with fever listed among general symptoms including fatigue, increased appetite, weight loss, and heat intolerance 1.
Hypersensitivity Reactions
Fever can also occur as part of hypersensitivity reactions to inactive ingredients in levothyroxine formulations, though hypersensitivity to levothyroxine itself is not known to occur 1. These reactions may include fever along with:
- Urticaria, pruritus, skin rash
- Gastrointestinal symptoms
- Arthralgia and serum sickness
- Wheezing
Clinical Implications
At therapeutic doses in properly monitored patients, fever should NOT occur. If a patient on levothyroxine develops fever, consider:
- Overtreatment: Check TSH and free T4 levels immediately
- Hypersensitivity reaction: Evaluate for other allergic symptoms
- Unrelated cause: Most commonly, fever is due to infection or other medical conditions unrelated to levothyroxine
Important Caveat
The presence of fever in a patient taking levothyroxine should prompt evaluation for thyroid storm if accompanied by tachycardia, altered mental status, or cardiovascular instability 2. However, in the vast majority of patients taking appropriately dosed levothyroxine, fever indicates an alternative diagnosis rather than medication effect.
The key distinction: fever from levothyroxine represents a sign of excessive dosing or hypersensitivity, not a side effect of appropriate therapeutic use.