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When Your Thyroid Levels Are Fluctuating

Risk Factors, Reasons and Solutions for Changes in TSH, T4 and T3 Levels


Updated June 10, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

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

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

What could account for thyroid levels that are frequently changing? Are you doing something that may be affecting your thyroid levels?

Here are some of the most 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. It happens more than you may believe. I have heard from many thyroid patients who discovered they were quite 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 the TSH/T4/T3 connections to dosage, and their confusion can result in dosage errors. I have heard from a number of patients whose former doctors, physician assistants and nurse practitioners told them "your TSH is really high, so that means you need to have your dose lowered." When challenged, some of these practitioners realized their mistake, but in some cases, they defended 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 medications in general, and thyroid drugs are particularly susceptible. So, mail-order medications that sit in hot trucks and mailboxes for hours and days, drugs that sit in hot cars or storage in hot homes without air conditioning can all have a fairly quick and dramatic effect on potency of medications. (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 there are errors in lab results. Samples can be degraded or switched, numbers transcribed, 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.

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