Why Are My Thyroid Hormone Levels Fluctuating?

If you're experiencing fluctuating thyroid hormone levels, you're not alone. Here are some common reasons, as well as tips on resolving them.  

why thyroid levels fluctuate
Illustration by Brianna Gilmartin, Verywell

Potency Fluctuations in Your Medicine

If you've 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.

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.

A good solution to avoid this problem is 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. 

Is Brand Name Levothyroxine Better Than the Generic Version?

Timing of When and How You Take Your Pill

If you're 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. 

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, as healthy, fiber intake can affect thyroid medication absorption.

Ultimately, consistency is what you should strive for, in terms of how you take your thyroid hormone drug. If you plan to change the way you take your thyroid medication, make sure you clear it with your doctor first. 

Dosage Errors

Pharmacy or doctor prescription errors do occur, meaning you may receive the incorrect dosage of your thyroid hormone replacement medication. One important tip is to always double check your medication—look at the label and the actual pills and make sure you're getting the dosage your doctor prescribed and the drug that was prescribed.

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.

Thyroid Goiter and Thyroid Nodules

Examples of foods that are "goitrogenic," include:

  • Brussels sprouts
  • Kale
  • Soy
  • Rutabaga
  • Turnips
  • Kohlrabi
  • Radishes
  • Cauliflower
  • African cassava
  • Millet
  • Babassu (a palm-tree coconut fruit popular in Brazil and Africa) 
  • Cabbage

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. 

Hormone 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. 

This is because estrogen increases a particular protein that binds thyroid hormone to it, making the thyroid hormone partially inactive—so, 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 (for example, 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 very partial list of medications that may affect thyroid levels includes certain cholesterol-lowering drugs, corticosteroids, lithium, and amiodarone.

Changing Course of Your Thyroid Disease

Hashimoto's Thyroiditis

Imagine this scenario in Hashimoto's thyroiditis: A person was diagnosed with autoimmune Hashimoto's disease a year ago, prescribed thyroid hormone, and on a six-week return visit had a TSH level of 2 mill-international units per liter (which is within the normal range). The person returns in a year, for a TSH recheck, and his TSH level is now elevated to 6.0 milli-international units per liter. 

This increase likely reflects the progression of the autoimmune process. In Hashimoto's thyroiditis, as thyroid antibodies further attack the thyroid gland, it's less and less able to produce thyroid hormone on its own. Therefore, T4 and T3 levels drop and TSH rises.

What is Hashimoto's Thyroiditis?

Graves's Disease

This same process works in the reverse with Graves's disease, where the same dose of antithyroid drugs that kept you in the normal range six months ago may now leave you hyperthyroid, as your thyroid becomes even more overactive.

In some cases, after months or more on antithyroid drugs, people with Graves' disease go into remission. In this case, their antithyroid drug dose can be decreased or even eliminated at times.

Coping with Graves' Disease and Hyperthyroidism

Thyroiditis After Pregnancy

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. However, with this fluctuation, thyroid hormone replacement drug dosages will 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.

If you do change your pattern or type of thyroid hormone drug intake, be sure to get your TSH retested no more than six to eight weeks later to determine whether a dosage adjustment is needed. 

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