Does Levothyroxine Cause Constipation? What to Check First

Levothyroxine is not a known cause of constipation. Its official label does not list it, and when the drug does upset the gut, it is usually because the dose is too high, which speeds things up rather than slowing them down. If you are constipated on levothyroxine, the more useful question is what else is going on: a dose that is still too low, a supplement or another medicine, or everyday habits.

Does levothyroxine cause constipation?

Not as a recognized side effect. The FDA prescribing information for Synthroid says adverse reactions to levothyroxine are "primarily those of hyperthyroidism due to therapeutic overdosage" [C1]. The gastrointestinal effects it lists are diarrhea, vomiting, abdominal cramps and changes in liver tests. Constipation is not on the list [C1].

That makes sense physiologically. Thyroid hormone speeds up gut movement. People with an overactive thyroid tend to have faster transit and diarrhea, while people with an underactive thyroid tend toward constipation [C2]. Levothyroxine replaces the hormone your thyroid is not making, so at the right dose it moves your gut back toward normal, and at too high a dose it pushes it toward looser, more frequent stools.

So if you notice constipation on levothyroxine, it is a clue worth following up. Most often it points away from the tablet itself. For a wider look at which symptoms come from the drug and which come from the dose, see levothyroxine side effects: real vs imagined.

Why does hypothyroidism cause constipation in the first place?

Thyroid hormone acts directly on the muscle cells of the digestive tract. When levels are low, the muscular waves that push stool along become weaker and slower, and constipation is one of the most frequent digestive signs of hypothyroidism [C2]. NIDDK lists hypothyroidism among the hormone conditions that can cause constipation [C8].

The good news is that this slowdown responds to hormone replacement. In a small 1984 study, gut transit time shortened significantly once hypothyroid patients were given thyroid replacement [C3]. If you were constipated before your diagnosis, it is reasonable to expect gradual improvement as your dose is set correctly. Our guide to constipation and hypothyroidism covers that side of the story in more detail.

Could your dose still be too low?

This is the first thing to rule out. If constipation started before your diagnosis and has not improved, or came back after a period of feeling well, the dose you are absorbing may not be enough. The label says that persistent signs of hypothyroidism despite an apparently adequate dose may reflect poor absorption, missed doses, drug interactions, or a mix of these [C1].

A TSH blood test answers the question. For adults, the label advises checking TSH 6 to 8 weeks after any dose change, and every 6 to 12 months once the dose is stable, or sooner if your symptoms change [C1]. Levothyroxine can take 4 to 6 weeks to reach its full effect, so constipation in the first weeks after starting or changing a dose may simply mean it has not caught up yet [C1].

Do not change your dose on your own. Bring the symptom to your doctor, ask whether a TSH check is due, and let the result guide any adjustment.

Which supplements and medicines cause constipation on levothyroxine?

Several common products do two things at once: they slow the bowel, and they reduce how much levothyroxine you absorb. That can create a loop where the supplement constipates you directly and also leaves you slightly under-replaced.

• Iron. NIDDK lists iron supplements among products that can make constipation worse [C8]. Iron also binds levothyroxine. In a 1992 study of 14 people on a stable dose, taking 300 mg of ferrous sulfate together with levothyroxine for 12 weeks raised average TSH from 1.6 to 5.4 mU/L, and 9 of the 14 had more hypothyroid symptoms [C5].
• Calcium. Calcium-containing antacids are on NIDDK's list of constipating products [C8]. Calcium carbonate and calcium citrate are also among the products shown to reduce levothyroxine absorption [C4].
• Other medicines. NIDDK also names opioid (narcotic) pain medicines, anticholinergics and antispasmodics, anticonvulsants, calcium channel blockers, diuretics, Parkinson's medicines and some antidepressants [C8].

The label advises taking levothyroxine at least 4 hours before or after calcium carbonate, ferrous sulfate and similar binders [C1]. Our calcium, iron and levothyroxine guide shows how to build that into a day.

Possible reasons for constipation on levothyroxine

Use this table to narrow it down before your next appointment.

Possible reason	Clue	What to check or ask
Dose still too low	Constipation plus tiredness, feeling cold or other hypothyroid symptoms; no improvement since diagnosis	Ask whether a TSH check is due (6 to 8 weeks after any dose change) [C1]
Iron supplement	Started or increased iron; dark stools, stomach upset	Ask about timing at least 4 hours from levothyroxine and whether another form suits you better [C1][C5][C8]
Calcium supplement or antacid	Daily calcium, a multivitamin with calcium, or frequent antacids	Ask about spacing from your tablet and whether the dose is needed [C1][C4][C8]
Fiber supplement timing	Started psyllium, bran or a fiber powder near your tablet time, or take it at varying times	Keep the timing the same every day and tell your doctor before your next TSH [C6][C7]
Other medicines	New opioid painkiller, anticholinergic, antidepressant, blood pressure or seizure medicine	Ask your pharmacist whether any of your medicines can cause constipation [C8]
Diet, fluids, activity	Low intake of fruit, vegetables and whole grains; little water; long sitting periods	Build toward 22 to 34 g of fiber a day, slowly, with enough fluids [C8][C9]
Brand or formulation change	Symptoms began after a pharmacy switch to a different tablet	Ask whether a TSH check makes sense after the change [C1][C4]
Something unrelated to the thyroid	IBS, celiac disease, pregnancy, travel, pelvic floor problems, or warning signs (see below)	See your doctor, especially if any red flag applies [C8]

Fiber helps constipation, but does it affect levothyroxine?

Fiber is one of the most useful tools for constipation. NIDDK suggests adults aim for 22 to 34 g of fiber a day depending on age and sex, add it gradually, and drink enough liquids to help it work [C9].

The catch is that fiber can interfere with levothyroxine. The label states that dietary fiber may bind levothyroxine and lower its absorption [C1]. In a 1996 report, people who needed unusually high doses had lower TSH or needed less levothyroxine once their fiber supplement was stopped, and wheat bran bound levothyroxine in lab tests [C6]. Yet a 1998 study in 8 volunteers found no detectable drop in absorption when psyllium or calcium polycarbophil was taken at the same time as levothyroxine [C7]. A 2017 review concluded the effect of fiber is not yet fully understood [C4].

The practical answer is consistency. Your dose is set around how you live, so a sudden change in fiber, or a fiber supplement taken at a different time each day, can shift how much you absorb. Many people take a fiber supplement several hours after their tablet (our fiber and levothyroxine guide uses a 4-hour gap). If you make a big change in your fiber intake, mention it to your doctor so it can be taken into account at your next TSH check.

Can a brand or formulation change cause constipation?

It is unlikely to cause constipation directly. The label does describe rare reactions to inactive ingredients, but the digestive symptoms it lists are abdominal pain, nausea, vomiting and diarrhea, not constipation [C1]. It also states that hypersensitivity to levothyroxine itself is not known to occur [C1].

A switch can matter indirectly. Levothyroxine has a narrow therapeutic index, so small differences in how much you absorb can be clinically meaningful [C4]. If constipation and other hypothyroid symptoms began after your pharmacy changed your product, it is reasonable to ask whether a TSH check makes sense. Our article on generic vs brand levothyroxine explains when a switch matters.

When to talk to your doctor

See your doctor if constipation does not improve with self-care, or if you have a family history of colon or rectal cancer [C8]. NIDDK advises seeing a doctor right away if constipation comes with any of these [C8]:

• bleeding from the rectum or blood in your stool
• constant pain in your abdomen
• being unable to pass gas
• vomiting
• fever
• lower back pain
• losing weight without trying

These signs are not explained by thyroid hormone levels and need their own assessment.

Thyra can help you gather the evidence for that conversation. Its medication timing feature plans your coffee, breakfast and supplement windows around your levothyroxine, and its symptom tracking looks for patterns once you have 7 or more tracked days.

Frequently asked questions

Can levothyroxine make you constipated at first?

Constipation is not a listed side effect of levothyroxine [C1]. In the first weeks, though, the drug has not reached its full effect yet (this can take 4 to 6 weeks), so constipation from hypothyroidism may simply not have eased [C1]. If it persists after your first follow-up TSH, raise it with your doctor.

Does too much levothyroxine cause constipation or diarrhea?

Too much levothyroxine is linked with diarrhea, not constipation. The label lists diarrhea, vomiting and abdominal cramps among the effects of an excessive dose [C1], and an overactive thyroid speeds up gut transit [C2].

Does constipation mean my levothyroxine dose is too low?

It can, but not always. Constipation is a common sign of hypothyroidism [C2], so a dose that is too low is worth ruling out with a TSH test [C1]. Iron, calcium, other medicines, low fiber and IBS can cause it at any TSH level [C8].

Can I take a laxative or stool softener with levothyroxine?

Ask your pharmacist which product fits you and how to time it. Some products that relieve or cause digestive symptoms, such as calcium-containing antacids, also lower levothyroxine absorption and need spacing from your tablet [C1][C4].

Is psyllium safe to take with levothyroxine?

Psyllium is widely used for constipation. Studies disagree on how much fiber affects levothyroxine absorption [C4][C6][C7], so the safest habit is to take it at the same time every day, several hours from your tablet, and tell your doctor when you start.

Bottom line

Levothyroxine is not a recognized cause of constipation; too high a dose leans toward diarrhea. If you are constipated on it, ask your doctor whether a TSH check is due, review your iron, calcium and other medicines with your pharmacist, and build fiber and fluids up gradually with consistent timing around your tablet. Any red flag, such as blood in your stool or unexplained weight loss, needs prompt medical attention.