What treatment looks like for an underactive thyroid

Treatment for an underactive thyroid — called hypothyroidism — almost always means taking a daily hormone replacement pill. The most common medication is levothyroxine, a synthetic form of the hormone your thyroid should be making on its own. Your doctor prescribes a starting dose based on your age, weight, and how low your thyroid hormone levels are, then adjusts it over weeks or months until your blood tests show the right level.

The goal is to bring your thyroid hormone levels back to normal so your body's metabolism, energy, and temperature regulation work properly again. For most people, this means taking one pill every morning on an empty stomach, usually 30 to 60 minutes before eating or drinking anything but water. Once you find the right dose, the medication is straightforward — you take the same amount every day for as long as you have the condition, which is usually lifelong.

Key Takeaways

  • Levothyroxine is the standard first-line treatment and works by replacing the hormone your underactive thyroid is not making.
  • Your doctor will start with a dose based on your weight and blood test results, then recheck your levels after 6 to 8 weeks and adjust if needed.
  • You take the pill on an empty stomach each morning, and certain foods and other medications can interfere with how your body absorbs it.
  • Most people need lifelong treatment, but the dose may change if you gain or lose significant weight, become pregnant, or start new medications.
  • If levothyroxine alone does not fully resolve your symptoms, your doctor may add a second hormone called liothyronine or switch to a different formulation.

How your doctor chooses the starting dose

Your doctor does not guess at the dose. They order a blood test that measures your TSH (thyroid-stimulating hormone) and sometimes your free T4 level — these numbers tell them how far below normal your thyroid hormone is. They then calculate a starting dose, usually between 25 and 100 micrograms per day, based on your body weight and the severity of the deficiency.

Older adults and people with heart disease often start at a lower dose because a sudden jump in thyroid hormone can strain the heart. Your doctor will schedule a follow-up blood test 6 to 8 weeks after you start, because it takes that long for the medication to reach a steady level in your blood and for your body to adjust. If your TSH is still too high or too low, they will increase or decrease your dose by 12.5 to 25 micrograms and retest again in another 6 to 8 weeks.

What to know about taking levothyroxine correctly

The timing and what you eat around your pill matter. Take levothyroxine on an empty stomach — ideally first thing in the morning — and wait at least 30 to 60 minutes before eating breakfast or taking other medications. Food, especially calcium and iron, can bind to the hormone and prevent your body from absorbing it fully, which means less of the medication reaches your bloodstream.

Several common substances interfere with levothyroxine absorption. These include calcium supplements, iron supplements, antacids that contain aluminum or magnesium, and some blood pressure medications. If you take any of these, space them at least 4 hours apart from your thyroid pill — take your levothyroxine in the morning and these other medications at lunch or dinner. Coffee and tea do not block absorption, but taking them with your pill may slightly reduce how much you absorb, so waiting an hour is safest.

If you miss a dose, take it as soon as you remember unless it is almost time for your next dose. Do not double up. Because levothyroxine stays in your system for about a week, missing one dose will not cause your symptoms to return immediately, but missing doses regularly will.

When your dose needs to change

Once you reach a stable dose, you will not need blood tests every month — usually your doctor rechecks once a year. However, certain life changes mean your dose may need adjustment. If you gain or lose more than 10 pounds, your doctor may want to retest because your body's hormone needs scale with body weight. Pregnancy increases thyroid hormone demand significantly, so women who become pregnant should tell their doctor right away so the dose can be raised before symptoms return.

Starting new medications can also affect how your body handles levothyroxine. Estrogen-based birth control, hormone replacement therapy, and some diabetes medications can change how much thyroid hormone your body absorbs or uses. Tell your doctor about any new prescriptions, and ask whether your thyroid dose needs checking. Similarly, if you switch brands of levothyroxine or move to a generic version, some people notice a difference — if your symptoms return after a brand change, mention it to your doctor, as they may need to adjust your dose slightly or switch you back.

What happens if levothyroxine alone is not enough

Most people feel better on levothyroxine alone, but some continue to have fatigue, weight gain, or brain fog even when their TSH is normal. If this happens, your doctor may add a second hormone called liothyronine (synthetic T3), which is the more active form of thyroid hormone. This combination therapy is not standard — most doctors try it only after levothyroxine alone has not worked — but some people report significant improvement.

Another option is switching from synthetic levothyroxine to desiccated thyroid extract, which comes from dried pig thyroid glands and contains both T4 and T3 hormones. This is less common because the hormone content varies between batches, making it harder to dose precisely, but some people prefer it. Your doctor will discuss these options only if standard treatment is not working and your blood tests confirm your dose is correct.

Side effects and when to contact your doctor

Levothyroxine is generally very safe, but taking too much can cause side effects. These include a racing or irregular heartbeat, tremors, anxiety, excessive sweating, and insomnia. These symptoms usually mean your dose is too high and your doctor needs to lower it. Contact your doctor if you notice any of these, especially if you also have chest pain or shortness of breath.

Allergic reactions to levothyroxine are rare but possible — signs include rash, itching, swelling of the face or throat, or difficulty breathing. If you experience any of these, stop taking the medication and seek immediate medical attention. Some people also report hair loss in the first few months of treatment, which is usually temporary and related to the body adjusting to normal hormone levels rather than a true side effect of the medication.

How long you will need treatment

Hypothyroidism is usually a permanent condition, meaning you will likely take levothyroxine for the rest of your life. The exception is postpartum thyroiditis, a temporary inflammation of the thyroid that happens after pregnancy — in these cases, thyroid function sometimes returns to normal after several months, and treatment can be stopped. Your doctor will monitor your thyroid function during recovery to know when it is safe to discontinue the medication.

For all other causes of hypothyroidism — including Hashimoto's disease, iodine deficiency, or thyroid removal — lifelong treatment is the standard. The good news is that once you find your stable dose, the routine becomes simple: one pill every morning, one blood test per year, and occasional dose adjustments. Most people on levothyroxine report that their energy, mood, and weight return to normal within a few months.

Frequently Asked Questions

Can I stop taking levothyroxine once I feel better?

No. Hypothyroidism is usually permanent, and stopping the medication will cause your symptoms to return within weeks as your hormone levels drop again. Your thyroid is not making the hormone, so you need to replace it every day. Your doctor will tell you if your condition is temporary — most are not.

What should I do if I cannot afford my levothyroxine?

Levothyroxine is one of the cheapest medications available, often costing just a few dollars per month even without insurance. Ask your doctor or pharmacist about generic versions, which are identical to brand-name Synthroid. Many pharmacies also offer discount programs or coupons that can lower the cost further.

Does diet affect how well levothyroxine works?

Certain foods and supplements can interfere with absorption if taken at the same time as your pill. Calcium, iron, soy products, and high-fiber foods are the main culprits. You do not need to avoid these foods — just take them several hours away from your levothyroxine dose, ideally at lunch or dinner while you take your pill in the morning.

Will levothyroxine cause weight loss?

Levothyroxine restores your metabolism to normal, which can help you lose weight that you gained because of the underactive thyroid. However, the medication itself does not cause weight loss — weight loss happens because your body is now burning calories at the normal rate. Diet and exercise still matter.

How often do I need blood tests once I start treatment?

Your doctor will retest 6 to 8 weeks after you start to see if your dose is right, then again after any dose change. Once you are stable, most doctors retest once per year. You may need more frequent testing if you are pregnant, have heart disease, or start new medications that interfere with levothyroxine.