Levothyroxine does not typically cause low blood pressure. In fact, the most common cardiovascular side effect is the opposite: increased blood pressure and heart rate from getting too much thyroid hormone. However, there are a few specific situations where levothyroxine can lead to a drop in blood pressure, and they’re worth understanding if you’re experiencing this.
What Levothyroxine Usually Does to Blood Pressure
When levothyroxine pushes your thyroid hormone levels too high, the cardiovascular effects lean toward overstimulation. The FDA label lists increased pulse and blood pressure, palpitations, rapid heart rate, and arrhythmias as the primary cardiovascular side effects. These are essentially symptoms of having too much thyroid hormone in your system.
More specifically, excess thyroid hormone raises systolic blood pressure (the top number) while actually lowering diastolic blood pressure (the bottom number) and reducing resistance in your blood vessels. This creates a wider gap between your two blood pressure readings, called a widened pulse pressure. So while the overall picture is one of higher blood pressure, the pattern is more nuanced than a simple across-the-board increase.
When Levothyroxine Can Lower Blood Pressure
There are three scenarios where levothyroxine genuinely can cause low blood pressure, and each works through a different mechanism.
Undiagnosed Adrenal Insufficiency
This is the most medically significant scenario. Your adrenal glands produce cortisol, the hormone that helps maintain blood pressure and manage stress. Some people have both an underactive thyroid and underperforming adrenal glands without knowing it. When levothyroxine speeds up your metabolism, it also accelerates the rate at which your liver breaks down cortisol. If your adrenal glands can’t keep up with that increased demand, cortisol levels plummet. The result can be dangerously low blood pressure, sometimes severe enough to mimic septic shock. This is called an adrenal crisis, and it’s the reason doctors sometimes check cortisol levels before starting thyroid hormone replacement, particularly if there’s any suspicion of adrenal problems.
Signs that this might be happening include extreme fatigue that worsens rather than improves after starting levothyroxine, dizziness when standing, nausea, and blood pressure readings that drop instead of stabilizing. This combination is a red flag that something beyond thyroid function needs attention.
Allergic Reaction
Rarely, people have a hypersensitivity reaction to levothyroxine or one of its inactive ingredients (fillers, dyes, or binders that vary by manufacturer). Low blood pressure is listed as a symptom of an allergic reaction to levothyroxine tablets, and it would show up as feeling faint or dizzy. This type of blood pressure drop would happen relatively soon after taking a dose and would likely come with other allergic symptoms like skin changes, swelling, or difficulty breathing. Switching to a different brand or formulation often solves the problem, since the active ingredient is the same but the inactive ingredients differ.
Reduced Diastolic Blood Pressure From Overtreatment
This one is subtle. If your dose is slightly too high, the excess thyroid hormone relaxes your blood vessels and lowers the diastolic number (bottom reading). While your systolic pressure may stay normal or rise, a noticeably low diastolic reading can cause symptoms like lightheadedness, especially when standing quickly. You might not register as having “low blood pressure” overall, but you could feel like you do.
Your Underlying Condition Matters Too
Hypothyroidism itself changes your cardiovascular system in ways that are relevant here. An underactive thyroid tends to raise diastolic blood pressure, narrow the gap between your two readings, and slow your heart rate. Your body adjusts to operating in that state. When levothyroxine starts correcting your thyroid levels, those cardiovascular parameters shift. Diastolic pressure drops, blood vessels relax, and heart rate increases. During this transition period, some people feel lightheaded or notice lower blood pressure readings, even though the medication is doing exactly what it should. This is especially common in the first few weeks of treatment or after a dose increase.
Over time, levothyroxine treatment generally improves blood pressure. Research in women with subclinical hypothyroidism showed that 18 months of treatment normalized both systolic and diastolic blood pressure. The adjustment period, though, can feel unsettling.
Blood Pressure Medications and Levothyroxine
If you take blood pressure medications alongside levothyroxine, the interaction is worth watching. There are no direct drug-to-drug interactions between levothyroxine and common blood pressure medications like calcium channel blockers. However, as levothyroxine restores your thyroid function, your cardiovascular system changes. Blood pressure medication doses that were appropriate when you were hypothyroid may become too aggressive once your thyroid levels normalize. The blood pressure medication hasn’t changed, but your body’s response to it has. This can result in blood pressure dipping lower than expected, particularly during the first several months of thyroid treatment. Your prescriber may need to adjust your blood pressure medication as your thyroid levels stabilize.
What Low Blood Pressure on Levothyroxine Means
If you’re experiencing low blood pressure after starting or adjusting levothyroxine, the cause usually falls into one of a few categories: your body is adjusting to normalized thyroid levels, your dose needs fine-tuning, your blood pressure medications need recalibrating, or something less common (like adrenal insufficiency or an allergic reaction) is at play. Persistent dizziness, fainting, or blood pressure readings consistently below 90/60 that started after beginning levothyroxine are worth bringing to your prescriber’s attention, particularly if you’re feeling worse rather than better on the medication. A simple blood test checking both thyroid and cortisol levels can usually sort out the cause.