Will levothyroxine (Synthroid) be expelled unchanged in the stool?

Levothyroxine Absorption and Fecal Excretion

No, you will not "poop out" your Synthroid (levothyroxine) in any clinically meaningful way—the vast majority is absorbed in your small intestine and metabolized by your body, with only about 20% eventually eliminated in stool after extensive processing. 1

How Levothyroxine Is Absorbed and Processed

Levothyroxine is absorbed primarily in the jejunum and upper ileum of your small intestine, with absorption rates ranging from 40% to 80% of the oral dose. 1 This means the medication enters your bloodstream well before reaching your colon, where stool is formed.

• Once absorbed, levothyroxine circulates in your blood bound to plasma proteins (>99% protein-bound), including thyroxine-binding globulin, thyroxine-binding prealbumin, and albumin 1
• The medication has an exceptionally long half-life of approximately 6-7 days in patients with normal thyroid function 1, 2
• This long half-life means levothyroxine stays in your system for weeks, not hours—it takes 6-8 weeks to reach steady-state concentrations 2

What Actually Happens to Levothyroxine in Your Body

The primary fate of levothyroxine is metabolism through sequential deiodination, not fecal excretion. 1

• Approximately 80% of circulating T4 (levothyroxine) is converted to T3 (the active thyroid hormone) through deiodination in peripheral tissues 1
• The liver is the major site of degradation for both T4 and T3, with additional metabolism occurring in the kidneys and other tissues 1
• Thyroid hormones are also metabolized via conjugation with glucuronides and sulfates, then excreted into bile and undergo enterohepatic recirculation (reabsorption from the gut back into the bloodstream) 1

The Small Amount That Does Reach Stool

Only approximately 20% of the daily T4 dose is ultimately eliminated in feces, and this represents metabolized hormone that has already been processed by your body—not intact medication. 1

• A portion of conjugated (already-metabolized) hormone reaches the colon unchanged and is eliminated in stool 1
• This fecal elimination represents hormone that has undergone extensive metabolism and conjugation, not unabsorbed tablets 1
• The primary route of elimination is actually through the kidneys (urinary excretion), not the gastrointestinal tract 1

Factors That Can Impair Absorption

If you're concerned about whether your levothyroxine is being absorbed properly, several factors can reduce absorption—but these affect uptake in the small intestine, not whether intact medication passes through to stool. 3

• Taking levothyroxine with food (especially dietary fiber, soybeans, grapefruit, papaya, or coffee) significantly decreases absorption 1, 3
• Medications like proton-pump inhibitors, antacids, iron supplements, and calcium supplements can impair absorption when taken within 4 hours of levothyroxine 1, 3
• Gastrointestinal diseases that disrupt intestinal barrier integrity or impair gastric acidity (such as celiac disease, inflammatory bowel disease, or gastroparesis) can reduce bioavailability 3, 4, 5
• Age can decrease absorption over time 1, 3

Critical Pitfall to Avoid

Never assume you're "pooping out" your levothyroxine if your TSH remains elevated despite treatment—the issue is almost always inadequate absorption in the small intestine or insufficient dosing, not fecal loss of intact medication. 3, 5

• If you have persistent hypothyroidism symptoms despite taking levothyroxine tablets, consider switching to liquid or softgel formulations, which have superior absorption profiles in patients with malabsorption 3, 4, 5
• Always take levothyroxine on an empty stomach, 30-60 minutes before breakfast, with a full glass of water to optimize absorption 1, 6
• Wait at least 4 hours after taking levothyroxine before consuming iron, calcium supplements, or antacids 1, 6