11 Reasons Why Your Thyroid and TSH Levels Are Fluctuating

Understanding Why TSH, T4 and T3 Levels Fluctuate

Female doctor discussing test results with patient
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One of the more common questions of patients taking thyroid hormone replacement medications is why, every time they have blood tests done, it seems their thyroid levels are fluctuating, often dramatically.

Most often, the changes are seen in thyroid stimulating hormone (TSH), T4 (thyroxine) and T3 (triiodothyronine).

What causes low TSH, or accounts for frequently changing thyroid levels? Could something you do affect your test results?

Here are some common factors that can cause your thyroid levels to fluctuate.

1. 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 medications. But sometimes, 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 of thyroid hormone replacement medication may be increased. (The dosage of antithyroid drugs might be reduced.)

To better understand these relationships, read: Understanding Low TSH, High TSH: Why a Low TSH Means Your Doctor Lowers Your Dose, And Other Confusing Issues.

2. Dosage Errors

Pharmacies make mistakes. I have heard from many thyroid patients who discovered they were 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. I have heard from too many patients whose former medical professionals said, mistakenly, that "your TSH is really 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 refilled prescription or different pharmacy in the time since your last thyroid tests were run, this may explain why levels have changed.

Thyroid hormone replacement drugs can fluctuate in terms of their potency and yet still be sold within Food and Drug Administration guidelines. In fact, the federal guidelines dictate that levothyroxine drugs need to be within 95% to 105% of stated potency. That means, a 100 mcg dosage pill can be considered potent, even while delivering anything from 95 to 105 mcg. of 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 having refills come 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 trucks, cars, mailboxes, or stores/homes without air conditioning can all lose potency due to the heat, quickly. (For more information, read Warning: Hot Temperatures May be Hazardous to Your Drugs: Your Medication Can Be a Casualty of Heat Waves and Power Outages.)

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.

4. Laboratory Changes, Mix-ups 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 earlier 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 causes of low TSH test results are lab errors. Samples can be degraded or switched, numbers transposed, etc. 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.

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."

You can read more about Dr. Shames' concerns in this article on Tests for Thyroid Testing and Evaluation.

5. Timing of When and How You Take Your Pill

If you are taking your thyroid medication at different times each day, you may be sometimes taking your thyroid hormone on an empty stomach, and sometimes with or after having food. Taking thyroid hormone with 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. 

If you want to ensure the best possible absorption, you'll want to take your thyroid medication 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 keep at least three to four hours between taking thyroid medication and taking any calcium or iron supplements. (That also goes for calcium-fortified juices.) 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.

And if you plan to change the way you take your thyroid medication, make sure you clear it with your practitioner first; 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.

6. 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 in slowing down and disabling the thyroid, ultimately causing -- or worsening existing -- hypothyroidism. If you still have a thyroid, you need to be more concerned about not over consuming 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) cabbage and kale 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.

7. 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.

8. Hormonal Fluctuations

Taking estrogen in any form, whether as hormone replacement therapy or in birth control pills, can affect 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 T-4 levels. For a woman without a thyroid gland in particular, this can increase the dosage requirement slightly, as there is no thyroid to compensate.

Menopause, with its fluctuating hormone levels, can also impact your TSH levels. (Read The Menopause Thyroid Connection.)

9. Pregnancy

The intense surge in estrogen during early pregnancy can increase your TSH and your body's need for thyroid hormone. It's particularly important for a woman to have her 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. See Pregnancy & Thyroid Disease Information Center.)

10. 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 have the potential to increase or decrease 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.

11. 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 sort of 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 still hyperthyroid, as the 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.


Levothyroxine Sodium Tablets: USP Monograph 

Braverman, MD, Lewis E., and Robert D. Utiger, MD. Werner and Ingbar's The Thyroid: A Fundamental and Clinical Text. 9th ed. Philadelphia: Lippincott Williams & Wilkins (LWW), 2005.

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