Ten Things You Didn’t Know About Synthroid

two prescription bottles with pills next to them
Dr. Sharon Orrange
Dr. Orrange is an Associate Professor of Clinical Medicine in the Division of Geriatric, Hospitalist and General Internal Medicine at the Keck School of Medicine of USC.
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Synthroid, one of the most commonly prescribed brand-name medications in the U.S., often requires lifelong treatment for hypothyroidism. For a medication that you may be paying a little more for and is a lifetime commitment, here are some lesser-known points about Synthroid.

1.  What is it?

Synthetic thyroxine (T4) comes as Synthroid or the generic levothyroxine. T4 is what is known as a prohormone that is deiodinated in our tissues to form T3, the active thyroid hormone. There is an advantage to taking the prohormone T4 which is that your own physiologic mechanisms control the production of an active hormone (T3).

2.  Synthroid vs levothyroxine?

There has been considerable controversy about their bioequivalence. Because there may be subtle differences in bioavailability between Synthroid and levothyroxine, it is preferable to stay with one formulation when possible. The American Thyroid Association and The Endocrine Society recommend that patients remain on the brand Synthroid if that was initially prescribed. If you are on generic levothyroxine try and stay with the same generic manufacturer; your pharmacy can help you with that.

3.  What’s the right dose of Synthroid to start on?

The average replacement dose of T4 in adults is approximately 1.6 mcg/kg per day which would be 112 mcg per day in a 70-kg adult, but this is not a one-size-fits-all recommendation.

4.  How do I take it? This matters.

Synthroid should be taken on an empty stomach, ideally an hour before breakfast. Another option is to take it at bedtime if you’ve had nothing to eat for several hours. Synthroid or levothyroxine should not be taken with other medications that interfere with their absorption, such as iron or calcium supplements or proton pump inhibitors (omeprazole, Nexium, etc).

5.  What blood tests for monitoring do I need?

Patients who are treated with T4 usually begin to improve within two weeks, but complete recovery can take several months. Although symptoms may begin to resolve after two to three weeks, steady-state TSH (thyroid stimulating hormone) concentrations are not achieved for at least six weeks. A TSH blood test should be measured six weeks after starting. Once the values of TSH in patients with primary hypothyroidism return to the reference range, periodic monitoring is warranted.

6.  What is my goal TSH on Synthroid?

Yes, there is plenty of controversy here, so try to rely on science. Most laboratories have used values of a normal TSH to be less than 4.5 to 5.0 mU/L but some argue that the upper limit of normal range should be reduced to 2.5 mU/L. This is because most “normal thyroid” volunteers have TSH blood values between 0.4 and 2.5 mU/L. The aim should be to keep TSH within the normal reference range (approximately 0.5 to 5.0 mU/L) but if you continue to have hypothyroid symptoms, it is reasonable to talk to your doc to increase the dose of Synthroid, and aim for a serum TSH value in the lower half of the normal range (0.4-2.5).

7.  I’m on too much Synthroid but I don’t mind the weight loss and how I feel, is that ok?

Overreplacement (taking too much Synthroid so your TSH is too low) is a bad idea and I see it all the time. The main risk is atrial fibrillation, which occurs three times more often in patients who are on too much Synthroid replacement. You may also have accelerated bone loss (and thus fractures).

8.  Do I need just Synthroid (T4) or do I need combination T4 and T3 therapy?

Again, another area of controversy. Here is what the science shows. Some folks remain symptomatic in spite of Synthroid or levothyroxine replacement and having normal serum TSH concentrations. In fact, in people taking Synthroid and with normal TSH values, 9-13% more still report impaired psychological well being compared to “normal thyroid” people. Years of studies have been done to see if people benefit from a combination of T3 (Cytomel) and T4 instead of just T4 (Synthroid) and almost all studies show that combination T4-T3 therapy does not appear to be superior to Synthroid monotherapy for the management of fatigue, bodily pain, anxiety, depression or quality of life.

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9.  What about switching from Synthroid to desiccated thyroid extract like Armour Thyroid?

Again, many studies have compared Synthroid to desiccated thyroid extract and revealed no differences in symptoms and neurocognitive measurements between the two groups.

10.  What messes with my Synthroid?

Many things can. Women taking estrogen therapy may need more Synthroid so a TSH blood test should be checked 12 weeks after starting estrogen therapy. A high-fiber diet can interfere with the absorption of Synthroid, and know that coffee reduces the absorption of Synthroid by about 30 percent.

So there’s that.

Dr O.

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