Levothyroxine is a synthetic thyroid hormone used primarily to treat hypothyroidism, a condition where your thyroid gland doesn’t produce enough hormone on its own. It’s the most commonly prescribed medication for an underactive thyroid and is also used after thyroid cancer surgery and to manage certain types of thyroid enlargement. The drug is chemically identical to the T4 hormone your thyroid naturally produces, so your body processes it the same way it would its own supply.
How Levothyroxine Works in Your Body
Your thyroid gland normally releases a hormone called T4, which acts as a raw material. Enzymes throughout your body then convert T4 into T3, the active form that regulates your metabolism, energy levels, heart rate, and body temperature. When your thyroid can’t keep up with demand, levothyroxine steps in as a direct replacement. It’s biochemically and physiologically indistinguishable from the T4 your body makes on its own, so once absorbed, it follows the exact same conversion pathway to T3.
Hypothyroidism: The Most Common Use
The vast majority of people taking levothyroxine have hypothyroidism. This can stem from several causes: Hashimoto’s thyroiditis (an autoimmune condition that gradually damages the thyroid), surgical removal of all or part of the thyroid, radioactive iodine treatment, radiation therapy to the head or neck, or certain medications that suppress thyroid function. Some people are born without a fully formed thyroid gland, which is called congenital hypothyroidism and is typically caught through newborn screening.
Without enough thyroid hormone, you may feel persistently fatigued, gain weight despite no change in eating habits, feel unusually cold, notice dry skin or thinning hair, or experience brain fog and sluggish thinking. Levothyroxine corrects the underlying hormone deficit, and most of these symptoms improve gradually over weeks to months once the dose is dialed in.
Levothyroxine is also prescribed for subclinical hypothyroidism, a milder form where blood tests show a slightly elevated thyroid-stimulating hormone (TSH) level but T4 levels are still in the normal range. Not everyone with subclinical hypothyroidism needs treatment, but your provider may recommend it based on your symptoms, TSH level, and other health factors.
Thyroid Cancer Management
After surgery for differentiated thyroid cancer (the most common type), levothyroxine serves a dual purpose. It replaces the hormone your body can no longer make, and it suppresses TSH, the pituitary hormone that tells thyroid cells to grow. TSH stimulates both normal thyroid tissue and thyroid cancer cells, so keeping TSH levels low helps prevent cancer recurrence and slows any remaining cancer cell growth.
The target TSH level for cancer patients is generally lower than for someone with standard hypothyroidism. Research shows that when TSH rises above 2 mIU/L, the risk of thyroid cancer recurrence and death increases. Providers aim to keep TSH below that threshold while balancing the side effects that can come with higher doses of levothyroxine. This suppression therapy is typically a long-term commitment, sometimes lifelong, with the target adjusted over time based on how the cancer responds.
Goiter and Thyroid Nodules
Levothyroxine is sometimes used to shrink or prevent the growth of goiters (an enlarged thyroid gland) and thyroid nodules. By supplying the body with thyroid hormone externally, the medication reduces the pituitary gland’s drive to produce TSH, which in turn decreases the signal telling the thyroid to grow. This use applies to several conditions, including multinodular goiter and chronic lymphocytic thyroiditis.
Why Levothyroxine Matters During Pregnancy
T4 is essential for fetal brain development, especially during the first trimester before the baby’s own thyroid begins functioning. Women already on levothyroxine typically need a dose increase of 20 to 30 percent as soon as pregnancy is confirmed. A practical way to accomplish this is to take two extra doses per week on top of the usual daily schedule. TSH levels are monitored closely throughout pregnancy, since the body’s demand for thyroid hormone rises significantly as the pregnancy progresses.
How to Take It for Best Absorption
Levothyroxine should be taken on an empty stomach, at the same time each day. Most people take it first thing in the morning, then wait 30 to 60 minutes before eating breakfast or drinking coffee. Coffee in particular can interfere with absorption if consumed too soon. The alternative is taking it at bedtime, at least three to four hours after your last meal.
Several common supplements and medications reduce how much levothyroxine your body absorbs. Calcium, iron, and aluminum-containing antacids are among the most well-known culprits, along with proton-pump inhibitors and high-fiber foods taken at the same time. If you take any of these, spacing them at least four hours apart from your levothyroxine helps ensure you’re getting the full dose.
Finding the Right Dose
The average full replacement dose for an adult is about 1.6 micrograms per kilogram of body weight per day. For someone weighing around 155 pounds (70 kg), that works out to roughly 100 to 125 mcg daily. Younger, otherwise healthy adults who’ve been hypothyroid for only a short time can often start at the full replacement dose right away.
Older adults and people with heart conditions start much lower, typically at 12.5 to 25 mcg per day, with increases every six to eight weeks. The same cautious approach applies to anyone who has been severely hypothyroid for a long time. Jumping to a full dose too quickly can put strain on the heart.
Once you’re on a stable dose, the goal is to keep your TSH within the reference range of 0.4 to 4 mIU/L. TSH is checked periodically, often every six to eight weeks during dose adjustments, and then once or twice a year once things stabilize. Because levothyroxine has a long half-life, it takes several weeks for a dose change to fully show up in blood work, so patience during the adjustment phase is important.
Signs Your Dose Is Too High
Taking more levothyroxine than your body needs pushes you into a state that mimics hyperthyroidism. Mild overtreatment can cause feeling hot or shaky, heart palpitations, difficulty falling asleep, and excessive sweating. If the excess is more significant, symptoms escalate to anxiety, mood swings, hand tremors, diarrhea, muscle weakness (particularly in the thighs and shoulders), unexplained weight loss, and a resting heart rate that’s noticeably faster than normal.
Beyond the day-to-day discomfort, long-term overtreatment carries two serious risks: heart arrhythmias and osteoporosis. Sustained excess thyroid hormone accelerates bone loss, increasing fracture risk, and places added stress on the cardiovascular system. This is why regular blood work matters even when you feel fine on your current dose.