11 Reasons You Have Fluctuating Thyroid Levels

Understanding Why Your TSH, T4, and T3 Levels May Change

Female doctor discussing test results with patient
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Do you wonder why your thyroid levels are fluctuating from test to test? What causes significant changes in your thyroid stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) levels? Are you doing something that is affecting your test results?

Here are some common reasons why your thyroid test levels may be fluctuating.

A Change in Your Medication Dosage

The most obvious cause of changes to your thyroid levels is a change in the dosage of your thyroid medication.

Sometimes, however, the relationship between your dosage changes and the test results can be confusing.

For example, low TSH often correlates to hyperthyroidism (excess thyroid hormone), and dosage of thyroid hormone replacement medication given to patients treated for hypothyroidism would usually be reduced, while antithyroid medications—used to treat hyperthyroidism—might be increased.

On the other hand, low T3/T4 or free T3/free T4 can indicate low levels of the two essential thyroid hormones, and dosage your thyroid hormone replacement medication may need to be increased, or your dosage of antithyroid drugs might be reduced.

The relationships may be confusing, but reading more about them can help.

Dosage Errors

Pharmacies make mistakes. Many thyroid patients discover they are overdosed or underdosed as a result of a pharmacy error. So one important tip: always double check your medication, look at the label, look at the actual pills and make sure you are getting the dosage your doctor prescribed and the drug that was prescribed.

A surprising source of errors is doctors themselves. Some simply do not understand how to dose thyroid medication. Unfortunately, it's far too common for uninformed practitioners to say "your TSH is high, so that means you need to have your dose lowered." When challenged, some of these practitioners realize their mistake, but in some cases, they defend their faulty information.

(If a practitioner is this misinformed, it's a key sign that you need a new thyroid doctor.)

3. Potency Fluctuations in Your Medicine

If you have started taking prescription thyroid hormone replacement medication from a newly refilled prescription or from a different pharmacy, this may explain why your levels have changed.

Thyroid hormone replacement drugs can fluctuate in terms of their potency and yet still be sold within Food and Drug Administration (FDA) guidelines. In fact, the federal guidelines dictate that levothyroxine drugs need to be within 95 percent to 105 percent of stated potency. That means a 100 mcg dosage pill can be considered potent, even while delivering anything from 95 to 105 mcg of the active ingredient.

While the potency tends to be fairly stable within a particular brand name or generic manufacturer, they do vary from brand to brand and manufacturer to manufacturer. Still, if you're stabilized on one brand, shifting to another brand—or being on generic levothyroxine and getting refills from different manufacturers—can cause some swings, based on the different potencies of each maker's drugs.

Another hitch? Hot weather can degrade thyroid drugs. Mail orders thyroid drugs that sit in hot delivery trucks, cars, mailboxes, or stores/homes without air conditioning can all quickly lose potency due to the heat exposure.

Some solutions:

  • Get larger supplies at one time. Some insurance companies will even encourage you to get three-month supplies via mail-order pharmacy services and discount the cost.​
  • Store your medications in a cool place, away from moisture (that means away from the bathroom) and heat.
  • If you are on a generic medication, work with your pharmacist to ensure that you always get medication from the same generic manufacturer. If that's not possible, consider switching to a brand name.

Laboratory Changes, Mixups, and Mistakes

Different laboratories processing blood tests may return slightly different results.

If you have fluctuating thyroid test results from one test result to the next, be sure to check with your practitioner to find out if the tests were sent to the same laboratory as previous tests. Test results from a new lab may account for substantially different results. In that case, it's worth retesting to ensure that the new results are accurate.

Sometimes lab errors cause erratic results. Samples can be degraded or switched, and numbers transposed. Some physicians have concerns about the accuracy of thyroid tests, in particular, the TSH test. Richard Shames, M.D., a California-based practitioner who has written a number of books on thyroid disease, feels that the handling of the samples for TSH tests may also result in inaccuracies because:

"...the blood that is drawn in the morning at almost every lab in the U.S. is usually not run through the machinery for analysis until that evening. During that time, your hormones—especially the important  TSH—may end up showing lower on your test result than is accurate for you. TSH is also a pituitary hormone that, according to the best standards, should be refrigerated properly once drawn. Most big labs have blood samples couriered to them. Are these kept at exact proper temperature? Hardly ever. TSH serum is rarely refrigerated."

So if you get results that simply don't make any sense, don't be afraid to ask the practitioner to confirm with a retest.

Timing of When and How You Take Your Pill

If you are taking your thyroid medication at different times each day, sometimes you may take your pill on an empty stomach, and sometimes with or after eating. Taking thyroid hormone with or after food may delay or reduce the drug's absorption by changing the rate at which it dissolves or by changing the stomach's acid balance, affecting your test results.

If you want to ensure the best possible absorption, you'll want to take your thyroid medication consistently, ideally first thing the morning, on an empty stomach, about one hour before eating and before drinking coffee. (Coffee can interfere with your medication absorption.) You may want to take your thyroid medication with vitamin C because studies show that it can improve absorption. Also, make sure to wait for at least three to four hours between taking thyroid medication and taking any calcium or iron supplements. (That also goes for calcium-fortified juices and cow's milk.) A high-fiber diet is also a factor. While healthy, fiber intake can affect thyroid medication absorption.

You may also wish to talk to your practitioner about taking thyroid medication at night because several studies have shown improved absorption at nighttime.

But ultimately, consistency is what you should strive for, in terms of how you take your drugs. If you're going to take your thyroid hormone with food, take it every day with food, consistently. Don't take it some days with food, some days without, or you're likelier to have erratic absorption, and it may be harder to regulate your thyroid levels.

If you plan to change the way you take your thyroid medication, make sure you clear it with your practitioner first. Also, be sure to get retested again no more than six to eight weeks after you've settled into your new pattern, to see if you need a dosage adjustment.

Eating Too Many Goitrogenic Foods

Certain foods can have what's known as a goitrogenic effect, or the ability to enlarge the thyroid and make it form a goiter. These foods can act like antithyroid drugs, slowing down your thyroid, and ultimately causing or worsening hypothyroidism. If you still have a thyroid, you need to be more concerned about not overconsuming these goitrogens in their raw form.

What foods are goitrogenic? Brussels sprouts, kale, soy, rutabaga, turnips, kohlrabi, radishes, cauliflower, African cassava, millet, babassu (a palm-tree coconut fruit popular in Brazil and Africa) and cabbage are all considered goitrogenic. Some experts believe that the enzymes involved in the formation of goitrogenic materials in plants can be destroyed by cooking, so thorough cooking may minimize some goitrogenic potential.

Change of Seasons

Many thyroid patients aren't aware that thyroid levels, and TSH in particular, can change along with the seasons. TSH naturally rises somewhat during colder months and drops back down in the warmest months. Some doctors adjust for this by prescribing slightly increased dosages during colder months and reducing dosage during warm periods. Most, however, are not aware of these seasonal changes, leaving patients increasingly hypothyroid during cold months or going through warmer months more hyperthyroid. This seasonal fluctuation can be more pronounced in older people, particularly those living in cold climates.

Hormonal Fluctuations

Taking estrogen in any form, whether as hormone replacement therapy or in birth control pills, can affect your thyroid test results. For example, some women taking supplemental estrogen may need to take more thyroid replacement hormone. Estrogen increases a particular protein that binds thyroid hormone to it, making the thyroid hormone partially inactive. Thyroid tests can end up showing falsely increased total T4 levels. For a woman without a thyroid gland, this can increase the dosage requirement as there is no thyroid to compensate.


The intense surge in estrogen during early pregnancy can increase your TSH and your body's need for thyroid hormone. It's particularly important to have your TSH tested periodically in early pregnancy, to ensure that dosages can be modified accordingly. TSH will frequently drop after delivery as well, in response to these shifts.

Herbs/Supplements/Drugs You Are Taking

Some herbal supplements can have an impact on thyroid function. Herbs such as the ayurvedic herb guggul, supplements such as tyrosine, products containing iodine such as vitamins, or kelp and bladderwrack supplements all have the potential to increase or decrease your thyroid function. Starting or stopping one of a number of prescription drugs can also affect thyroid levels. A partial list of medications that can affect thyroid levels include antidepressants, cholesterol-lowering drugs, corticosteroids, lithium, and amiodarone.

The Changing Course of Your Thyroid Disease

You may have been diagnosed with autoimmune Hashimoto's disease a year ago, prescribed thyroid hormone, gone back six weeks later, got rechecked and your TSH was 2. The doctor decided that your levels were fine and told you to come back in a year to be retested. This year's test shows your TSH at 5.7. This increase may reflect the progression of the autoimmune process. As thyroid antibodies further attack the thyroid, it can become less and less able to produce thyroid hormone on its own; therefore, T4 and T3 levels drop and TSH rises.

This same process works in the reverse with Graves' disease, where the same dose of antithyroid drugs that kept you in the normal range six months ago is now leaving you hyperthyroid, as your thyroid becomes even more overactive. In some cases, after months or more on antithyroid drugs, some Graves' disease patients also go into remission, so you find that your antithyroid drug dose can decrease or even be eliminated at times.

In addition, some women develop thyroiditis after pregnancy. For the majority of these women, the condition will resolve itself, meaning that over time, the thyroid will attempt to return to normal and blood test levels will reflect these changes. Drug dosages need to be changed accordingly.

A Word From Verywell

Careful management of your thyroid levels, as well as symptoms, is an essential part of your thyroid treatment. It's not only important to monitor your thyroid test results for changes, but for you and your practitioner to understand the reasons behind those changes, so they can be addressed.


Bahn, R., Burch, H, Cooper, D, et al. Hyperthyroidism and Other Causes of Thyrotoxicosis: Management Guidelines of the American Thyroid Association and American Association of Clinical Endocrinologists. Endocrine Practice. Vol 17 No. 3 May/June 2011.

Braverman, L, Cooper D. Werner & Ingbar's The Thyroid, 10th Edition. WLL/Wolters Kluwer; 2012.

Garber, J, Cobin, R, Gharib, H, et. al. "Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association." Endocrine Practice. Vol 18 No. 6 November/December 2012.