Synthroid is a synthetic hormone used to treat hypothyroidism, an enlarged thyroid gland or thyroid cancer, all conditions related to a malfunctioning thyroid gland.[1] If you are prescribed Synthroid, it will probably be under the prescription name Levothyroxine.

A healthy thyroid gland produces a hormone called thyroxine (T4) that regulates many functions in the body. When blood tests reveal that you are not producing enough thyroxine naturally, a doctor will usually prescribe a thyroid replacement medication for the rest of your life.

Your synthroid dosage will vary from the lowest dosage of 25 mcg to the maximum of 300 mcg. Your doctor will determine the best level to start you out at depending on your symptoms and blood work indicator. However, over the course of your lifetime, the dosage will almost certainly need to be adjusted from time to time.

How is Thyroid Disease Diagnosed?

What Causes Hypothyrodism?
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The state of your thyroid gland is revealed in routine blood work. If you look on your lab work, you will see something called TSH. This stands for Thyroid Stimulating Hormone. It is a measure that might indicate you have a problem with your thyroid gland.[1]

I say might because TSH is actually produced by the Pituitary Gland.[1] Based on fluctuations in this level, doctors are able to conduct more in-depth blood work to determine if the issue is actually a problem with your Thyroid, or something else.

The American Association of Clinical Endocrinologists recommends a TSH level between 0.3 and 3.0 mIU/L. This has been adjusted over the years and was recently reduced from a normal range of .5 to 5.0mIU/L.

However, the averages are just that, an average. Every situation is different and if your TSH level falls outside this range, it does not necessarily indicate you have a problem.

Further blood tests should be performed to test your T3 and T4 levels which are directly related to how the thyroid is functioning.[2]

You will need to work with your doctor to find the dose that is right for you. This means office visits for lab tests, especially during the first months of treatment.

What are T3 and T4 Levels?

The thyroid gland is small but manages a lot of vital functions in the body, primarily ther regulation of the use of energy.

The thyroid gland produces two hormones: called thyroxine, referred to as your T4 level on your lab work, and triiodothyronine indicated by T3. There is also another indicator called FT4 which refers to free thyroxine levels.

T4 - most thyroxine is attached to a protein called thyroxine-binding globulin.[2]

FT4 – less than 1% of T4 is not attached to thyroxine-binding globulin. Measuring how much T4 is free allows doctors to determine if there are abnormal amounts are present because of abnormal amounts of thyroxine-binding globulin.[2]

T3 - Most of the T3 in the blood is also attached to thyroxine-binding globulin. Less than 1% is unattached. T3 is the primary decision maker in how the body uses energy.[2]

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Symptoms of Hypothyroidism

What is Synthroid?
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Hypothyroidism is caused by an underactive thyroid gland and typically manifests itself in various ways. Here are some of the symptoms of a thyroid problem.[2]

  • Weight gain
  • Dry/Yellow skin
  • Hair loss
  • Swollen face, hands, legs, ankles, or feet
  • Feeling cold
  • Aches and pains in muscles or joints
  • Hoarse or raspy voice
  • Constipation
  • Heavy menstrual bleeding or irregular periods
  • Fatigue/Slower thinking, speech or movement
  • Memory loss
  • Depression
  • Enlarged thyroid gland
  • Fluctuations in cholesterol level
  • Slow heart rate
  • Infertility

However, just because you are not experiencing any of the symptoms does not mean you do not have an underactive thyroid. The blood tests that determine your T3 and T4 levels are key.

Side Effects of Synthroid

When you begin taking Synthroid, you may experience various symptoms as your body adjusts to the hormone replacement therapy. These adjustments can last anywhere from 3 months to 6 months, sometimes longer, and may require an adjustment of your dosage.

For instance, some people experience hair loss to some degree during the first few months taking the medication. However, your doctor will insist that is due to the hypothyroidism, not the medication.

However, if you look on the packaging for Synthroid, one of the side effects listed is “may cause hair loss in some individuals”.

Many people experienced with thyroid replacement hormone therapy say that initially you will shed some hair. The theory offered by some is that your “unhealthy” hair is being pushed out for newer hair to grow. While there is no medical data to support this, do not panic if you see yourself raining hair after you have been on the medication for a few weeks. Hair loss should moderate and eventually stop. If it does not, you may need to up or lower your dosage after consulting with your doctor.[3]

As I said, hair loss is a symptom of hypothyroidism, so it may be difficult to convince your doctor that the medication is causing you to shed hair. Only you know your body and if the shedding did not start until you started the medication, you know that you are one of the “some” that was referred to on the package insert of the medication.

Here are some other side effects that may occur when you start taking Synthroid, so you may need to adjust the dosage.[3]


A healthy thyroid gland is crucial to your overall health. Hypothyroidism primarily affects What is Synthroid?Credit: Opensourcewomen, although men should pay attention to their lab work or any of the symptoms listed here.

If you suspect you may have an issue, talk with your doctor. The first official indicator will be your lab work, however, you will be noticing symptoms beforehand, most likely fatigue or weight gain.

If you require Synthroid hormone replacement, it is not the end of the world. You will probably be on the medication for the rest of your life, but underactive thyroid gland is a manageable condition and the thyroid medication side effects are minimal once you get your dosage set.

On a positive note, the thyroid has been shown to heal itself and your TSH, T3 and T4 levels all returning to normal levels. Most changes can be traced back to a healthier eating and inclusion of more iodine in your diet, avoiding soy, exercise or taking various supplements.