Levothyroxine dosing starts with a weight-based calculation, then gets fine-tuned over several months using your TSH results as the primary guide. The standard full replacement dose for adults is 1.6 mcg per kilogram of body weight per day, which means a 70 kg (154 lb) person would typically need around 112 mcg daily. But that’s just a starting point. Your actual dose depends on your age, heart health, how elevated your TSH is, and how your body responds over time.
How the Starting Dose Is Calculated
For most adults with newly diagnosed hypothyroidism, the initial dose is based on body weight: 1.6 to 1.7 mcg per kilogram per day, depending on the formulation (tablets, capsules, or liquid). In practice, this gets rounded to the nearest available tablet strength. Someone weighing 68 kg (150 lb) might start on 100 or 112 mcg, for example.
Not everyone starts at the full calculated dose. Older adults, people with heart disease, and those with severe or long-standing hypothyroidism typically begin at a lower dose and work up gradually. The concern is that introducing too much thyroid hormone at once can strain the heart, causing palpitations or chest pain. For these patients, increases happen in small steps of 12.5 to 25 mcg every 6 to 8 weeks, with TSH rechecked each time.
The TSH Target for Most Adults
The goal of treatment is to bring your TSH into the normal reference range, generally 0.4 to 4.0 mIU/L. Most prescribers aim for the lower half of that range, somewhere between 0.5 and 2.5 mIU/L, where many patients feel their best. But “optimal” varies. The relationship between TSH and thyroid hormone levels is complex and nonlinear, differing by age and sex, so two people with the same TSH number can feel quite different.
For adults over 65 or 70, some clinicians accept a TSH on the higher end of normal. Pushing the TSH too low in older adults can increase the risk of irregular heart rhythms and bone loss, so a slightly higher target may be safer.
How Doses Get Adjusted Over Time
After starting levothyroxine or changing a dose, you need to wait before retesting. TSH should be rechecked no earlier than four weeks after a dose change, though the full effect on TSH may not be apparent until eight weeks. Testing too soon can give a misleading picture and lead to unnecessary adjustments.
When your TSH comes back above the target range, the typical adjustment is an increase of 12.5 to 25 mcg. If your TSH is below the target (meaning you’re getting too much), the dose is reduced by a similar increment. This cycle of adjust, wait, retest continues every 4 to 6 weeks until your TSH stabilizes in range and your symptoms improve. Once you’re stable, TSH is usually monitored once or twice a year.
Smaller adjustments (12.5 mcg) are preferred when your TSH is only slightly out of range. Larger jumps (25 mcg) make sense when your TSH is significantly elevated. The increments are intentionally small because levothyroxine is a potent medication, and even a modest change can shift your TSH considerably.
Subclinical Hypothyroidism: When TSH Is Only Mildly Elevated
If your TSH is above normal but your thyroid hormone levels are still in range, you have subclinical hypothyroidism. Treatment decisions here depend heavily on how high the TSH actually is. In randomized controlled trials, treating patients with a TSH below 10 mIU/L does not consistently improve symptoms or cognitive function. Most guidelines suggest treatment is not necessary unless the TSH exceeds 7 to 10 mIU/L, though individual factors like symptoms, age, and the presence of thyroid antibodies can shift that decision.
If treatment is started for subclinical hypothyroidism, the starting dose is typically lower than for overt hypothyroidism, often 25 to 50 mcg, since the thyroid is still producing some hormone on its own.
Special TSH Targets During Pregnancy
Pregnancy changes thyroid hormone demand significantly, and the TSH targets are tighter. The American Thyroid Association has recommended a TSH upper limit of 2.5 mIU/L in the first trimester and 3.0 mIU/L in the second and third trimesters. Women already on levothyroxine often need a dose increase of 12.5 to 25 mcg as soon as pregnancy is confirmed, with TSH rechecked every four weeks until the dose is stable.
Many women need 25 to 50 percent more levothyroxine during pregnancy than their pre-pregnancy dose. After delivery, the dose typically returns to its previous level.
TSH Suppression After Thyroid Cancer
Patients treated for thyroid cancer are a different situation entirely. Rather than simply replacing missing hormone, the goal can be to suppress TSH below normal levels to reduce the risk of cancer recurrence. For intermediate-risk thyroid cancer, a TSH below 0.5 mIU/L may be recommended. For low-risk and very low-risk papillary thyroid cancer, current guidelines advise against TSH suppression, since the cardiovascular and bone risks of keeping TSH chronically low outweigh the benefit when recurrence risk is already small.
Why Your TSH Might Not Respond as Expected
If your TSH stays stubbornly high despite dose increases, the problem may not be the dose itself but how much of the medication your body is actually absorbing. Levothyroxine is best taken on an empty stomach, ideally 30 to 60 minutes before eating, because many common foods and supplements interfere with absorption.
Coffee, soy products, and high-fiber foods all reduce absorption when taken at the same time as the medication. Calcium supplements are a particularly common culprit, causing a significant TSH increase in roughly 4 to 5 percent of patients who take them together. Iron supplements have a similar effect, raising TSH in about 7.5 percent of people. Even antacids containing aluminum hydroxide can nearly triple TSH levels, in one documented case pushing TSH from 2.6 to over 7 mIU/L. Separating these substances from your levothyroxine by at least four hours solves the problem in most cases.
Biotin Can Falsify Your TSH Results
One often-overlooked issue is biotin supplements, which are popular for hair and nail growth. Biotin at doses of 20 mg or higher can directly interfere with the lab assay used to measure TSH, producing falsely low TSH readings that mimic hyperthyroidism. In documented cases, patients taking 20 to 30 mg of biotin showed TSH values as low as 0.05 mIU/L, which returned to normal (around 1.4 mIU/L) within 48 hours of stopping the supplement. If you take biotin, stop it at least two to three days before any thyroid blood work. Most standard multivitamins contain biotin doses too low to cause this problem, but standalone biotin or “hair, skin, and nails” supplements often contain enough to skew results.