Why Your Thyroid Hormone Levels May Be Fluctuating

If you have thyroid disease, you may experience some fluctuations in your thyroid hormone levels from time to time. These fluctuations can occur as your thyroid disease progresses, but other factors, such hormonal changes, and medication variations, can alter your thyroid hormone levels as well, producing a variety of symptoms.

why thyroid levels fluctuate
Illustration by Brianna Gilmartin, Verywell

The Changing Course of Your Thyroid Disease

Thyroid disease can progress or stabilize as the years go on, either due to the natural course of the condition itself or to major changes triggered by the treatment (such as surgical removal of the thyroid gland).

Hashimoto's Thyroiditis

Hashimoto's thyroiditis often progresses over the first 10 years. After diagnosis and treatment are established, thyroid antibodies may continue to further attack the thyroid gland, making it less and less able to produce thyroid hormone on its own.

Therefore, even if you maintain the same treatment dose, your thyroxine (T4) and triiodothyronine (T3) thyroid hormone levels can drop, causing your thyroid-stimulating hormone (TSH) to rise in response. Overall, you may feel the symptoms of hypothyroidism, including fatigue, weight gain, and depression.

Graves's Disease

Graves' disease can also progress, especially in the first few years after diagnosis. In many instances, even when your treatment is at the right dose, your T3 and T4 levels may continue to rise, your TSH may fall, and you can develop symptoms of hyperthyroidism, such as the inability to concentrate, insomnia, and weight loss.

With Grave's disease, the opposite can happen, too. In some cases, after months or years of taking antithyroid medications, your condition can go into remission, your T3 and T4 levels may decrease (while your TSH rises), and you can develop symptoms of hypothyroidism.

Thyroiditis After Pregnancy

Some women develop thyroiditis after pregnancy. Usually, this is characterized by low thyroid hormone levels and high or low TSH, but high thyroid hormone levels with low or high TSH can develop as well.

Usually, postpartum thyroiditis will resolve itself and the thyroid hormones and TSH will eventually return to normal. However, during the period of thyroiditis, thyroid hormone replacement or antithyroid medications are usually needed to help control the symptoms. These medications can be decreased or discontinued if and when the condition improves.

Hormone Fluctuations

Taking estrogen in any form, whether as hormone replacement therapy or in birth control pills, can affect your thyroid test results.

Estrogen increases thyroxine binding globulin, a protein that binds to T3, making it partially inactive. This results in high TSH, low T3, and falsely increased total T4 levels.

Women taking supplemental estrogen or who do not have a thyroid gland may need to take more thyroid replacement hormone due to the interference of estrogen-containing medications. If you are hyperthyroid, the interaction of estrogen with your T3 may decrease your need for antithyroid medication.


Pregnancy can affect thyroid hormones in many ways, and the changes in these levels are more extreme if you already have a thyroid condition prior to becoming pregnant.

  • Without pre-pregnancy thyroid disease: In general, T3 and T4 tend to increase during pregnancy, and TSH decreases. This happens because human chorionic gonadotropin (HCG), a hormone produced during pregnancy, stimulates the production of T4 and T3.
  • If you were hyperthyroid before pregnancy: In this case, the effect of HCG can increase your T4 and T3, and decrease your TSH even more than usual during your pregnancy.
  • If you were hypothyroid before pregnancy: The surge in estrogen during early pregnancy can decrease your T3 and T4 levels and increase your TSH.

Medication Potency Differences

If you've started taking prescription thyroid hormone replacement medication from a newly refilled prescription or from a different pharmacy, your thyroid hormone blood levels may change.

Within the Food and Drug Administration (FDA) guidelines, thyroid hormone replacement medications can fluctuate in their potency.

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 though it delivers anywhere from 95 mcg to 105 mcg of the active ingredient.

While the potency tends to be fairly stable within a particular brand name or generic manufacturer, they can vary from one manufacturer to another. If you're stabilized, shifting to another brand or getting refills of generics from different manufacturers can cause some swings in your levels because of the different potencies of each product.

Depending on your condition, these potency variations can cause mild increases or decreases in your T4, T3, or TSH, as well as corresponding symptoms of hypothyroidism or hyperthyroidism.

If you are on a generic medication, work with your pharmacist to ensure that you always get medication from the same generic manufacturer or consider switching to a brand name to avoid this issue.

Also, keep in mind that prescription errors can occur. 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 drug and the dosage your doctor prescribed.

Thyroid Disease Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Old Woman

When and How You Take Your Pill

If you're taking your thyroid replacement or antithyroid medication at different times each day, you might not be consistent about taking it on an empty stomach as recommended. 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, ultimately affecting your thyroid hormone levels, your symptoms, and your test results.

If you want to ensure the best possible absorption of your medication, take your thyroid medication consistently. Ideally, you should take your thyroid medicine in the morning, on an empty stomach, about one hour before eating breakfast and drinking coffee. 

Also, make sure to wait for at least three to four hours between taking thyroid medication and taking any fiber, calcium, or iron-rich foods or supplements, as they can prevent you from absorbing your full dose of medication.

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

Other Medications and Herbs

Some herbal supplements and medications can have an impact on thyroid hormone levels, either by competing with the body's thyroid hormone activity, amplifying the effects of thyroid hormones, or altering medication absorption and activity.

  • Medications: Starting or stopping prescription drugs that you are taking for another condition besides your thyroid disease can affect your thyroid levels and your symptoms in a number of different ways. Some medications that may affect thyroid levels include certain cholesterol-lowering drugs, corticosteroids, growth hormone, lithium, and amiodarone.
  • Herbs: The Ayurvedic herb guggul, supplements such as tyrosine, products like kelp that contain iodine, and bladderwrack supplements are some of the naturopathic remedies that are known to increase or decrease thyroid function, alter test results, and produce a variety of thyroid related symptoms.

It is best to ask your pharmacist and doctor about potential interactions.

Goitrogenic Foods

Certain foods can have what's known as a goitrogenic effect, which is the ability to enlarge the thyroid and make it form a goiter, which interferes with the gland's function. These foods can act like antithyroid drugs, slowing down your thyroid and ultimately worsening hypothyroidism. While the effect can be subtle, they may lower your T4 and T3 levels, with a resulting increase in your TSH and symptoms of hypothyroidism.

Examples of foods that are goitrogenic include:

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

If you have thyroid disease, you need to be especially cautious to avoid overconsuming these goitrogens in their raw form.

Some experts believe that the enzymes involved in the formation of goitrogenic materials in plants can be destroyed by heat, so thorough cooking may minimize some goitrogenic potential.

Change of Seasons

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. 

A Word From Verywell

Careful management of your thyroid levels is an essential part of your thyroid treatment. There are a number of factors that can alter your symptoms and thyroid test results, and addressing these issues can help keep your thyroid levels stable.

If you notice a change in your symptoms, be sure to tell your doctor, who might want you to have your thyroid hormone levels retested, and may make an adjustment in your medication dosing.

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