Hypothyroidism is a condition classified by an underactive thyroid gland — when the thyroid does not produce enough hormones.

There are various treatments available depending on the cause of insufficient hormone production, but the standard of care is the same — initiating thyroid hormone replacement therapy.

To best understand the purpose of thyroid hormone replacement therapy, it helps to appreciate the role and interaction of T3 and T4 — the two primary thyroid hormones.

T3 and T4

The full name of T3 is triiodothyronine, and T4’s full name is tetraiodothyronine or thyroxine. T3 and T4 control your body’s metabolism. If you don’t have enough of these hormones, then your metabolism slows down.

Your metabolic rate dictates how quickly you process food, how fast your heart beats, how much heat your body produces — and even how quickly you can think. In essence, T3 and T4 are in charge of how your body uses energy.

T3 and T4 are not equal in strength; T3 is the more active hormone of the two. While T3 is stronger, taking synthetic T4 hormone has been considered the standard treatment for hypothyroidism.

The reason for this is because most of the T3 in our bodies actually used to be T4. When T4 hormones come into contact with other cells in the bloodstream, they give up an iodine atom to interact with those cells. When T4 loses an iodine atom, it becomes T3.

When this T4 to T3 conversion occurs, T3 then conveys the metabolic “message” to the other cells throughout the body. The benefit of taking only T4 therapy is that you’re allowing your body to perform some of the actions it is meant to do, which is taking T4 and changing it into T3. The half-life of T4 is also longer compared to T3 (7 days versus 24 hours), which means that it will stay for a longer time in your body after ingestion.

However, there is a growing consensus among the experts that providing a supplement of both T3 and T4 is beneficial for many patients who have had a total thyroidectomy (removal of the entire thyroid gland).

The Purpose of Thyroid Hormone Replacement Therapy

If you are prescribed a form of thyroid hormone replacement therapy, the goal is to compensate for the lack of hormone secreted by your thyroid gland. In most cases, you will take a daily dose of T4 (or T3 and T4) in a pill taken orally.

But it’s important to understand that every patient’s therapy may be different. There is no cookie-cutter dosage or treatment plan when it comes to thyroid hormone replacement therapy. How the body absorbs the hormones, along with the amount of hormones needed to help the body function properly, is very varied.

Your treatment plan will be individualistic. As such, you should expect some degree of adjustment when it comes to finding the dosage and form of therapy that works best for you.

Though synthetic T4 supplements have been the most prescribed form of thyroid hormone replacement therapy, there are a variety of forms, including animal thyroid supplements.

Synthetic T3 is also occasionally given as part of treatment, most often after thyroid surgery, when waiting for radioiodine ablation in cases of cancer, or because the entire thyroid has been removed.

Thyroid hormone replacement therapy is a very individualized treatment process, and it is highly effective when prescribed properly. The goal of thyroid hormone replacement, in most cases, is to normalize your thyroid-stimulating hormone (TSH) levels.

You and your doctor will discuss what treatment option will best alleviate your hypothyroid symptoms, allowing you to live a healthy, normal life.

Synthetic forms of the thyroid hormone T4 are generally called levothyroxine, and they are considered the standard treatment for hypothyroidism. Levothyroxine is considered a levoisomer, which simply means that the compound was produced by rotating at a specific wavelength but this has no impact on its biological function.

As such, levothyroxine mimics the natural hormone. Though this hormone is man-made, synthetic T4 hormones are very similar to the T4 that is produced and released by the thyroid gland.

Types of T4 Supplements

There are currently six FDA-approved brands of synthetic T4 supplements available:

  • Levo-T;
  • Levothyroxine Sodium;
  • Levoxyl;
  • Novothyrox;
  • Synthroid;
  • Unithroid.

Synthroid has been the most commonly prescribed brand of T4 for hypothyroidism but health insurers’ push to lower costs has required many patients to switch to the generic supplement, levothyroxine sodium.

The T4 supplement delivers a steady, prolonged dose of the needed hormone. When making a change in the formulation of your supplement, having your levels checks and adjustments in dosage may be necessary to assure your TSH levels remain in the desired range.

All the approved brands of T4 are considered essentially the same, and most importantly, provide favorable hormonal effects. In other words, there is no significant difference in their composition. However, that does not mean that these brands are exactly the same.

The bioavailability of a given brand at a given time after ingestion might be different. That’s why much of the endocrinology community — the American Association of Clinical Endocrinologists, the Endocrine Society, and the American Thyroid Association — believe that once you start with a brand, you should stick with it, if at all possible.

Determining Dosage

Finding your ideal T4 dosage is essential. The right dosage keeps hypothyroidism from interfering with your life. The wrong dosage can make it an even bigger problem than it was before you sought treatment.

Getting the right dosage is important, but don’t expect the dosage you start out with to be the dosage that you eventually stay with. Doctors often use weight as a guideline for determining dosage. Some clinicians use the formula of 1.6 micrograms of T4 for every 1 kilogram (or 2.2 pounds) of body weight for a starting dosage.

Others prefer a more conservative approach, starting patients at a very low dose (perhaps as low as 25 micrograms). Note that the 1.6 micrograms are considered a full replacement dose. This means that if part of your thyroid still functions properly, you won’t need this full dose because your body is continuing to make some of its own T4, in addition to the hormone supplied by the pill.

Because it’s common for dosages to change at the start of treatment, a doctor will likely monitor your thyroid-stimulating hormone (TSH) levels after two or three months (though some doctors may check as soon as four weeks) from your first day of treatment.

And since hormone replacement therapy is usually a lifelong treatment, you should get checked every year to make sure you’re taking the right dose if you’re on a stable dose. You should communicate with a doctor more frequently if your dose is being adjusted.

Doctors often err on the side of caution when prescribing starting dosages of T4 for a variety of reasons. For one, starting at a low dose and moving up lets your heart get used to the increased metabolism. Also, your doctor doesn’t want to risk inducing hyperthyroidism — a condition caused by high levels of thyroid hormones. You can learn more about the symptoms of over-treatment below.

Over-treatment Symptoms

Even with a moderate dose of T4, some patients are susceptible to over-treatment symptoms. Elderly patients with weaker hearts and people with heart arrhythmias (irregular heartbeat) are especially sensitive to thyroid hormone. Generally, doctors like to start with a slightly lower dose in these patients, in order to avoid or worsen irregular heartbeats.

Below are some of the symptoms of over-treatment:

  • feeling hot and sweating more than normal;
  • shaking (hand tremors);
  • heart palpitations;
  • having difficulty falling asleep;
  • having mood swings;
  • experiencing mental “fuzziness” (forgetfulness, loss of concentration);
  • experiencing muscle weakness;
  • losing weight;
  • menstrual irregularities.

If you experience any of these symptoms throughout the duration of your hypothyroidism treatment, talk to your doctor immediately. He or she will first check your blood tests, before deciding on a dose.

Taking Other Medications with T4

Because T4 is ingested, it must be absorbed from the gastrointestinal tract into the bloodstream. The medications and supplements listed below should NOT be taken at the same time as T4, as they can interfere with your body’s ability to absorb the hormone:

  • Aluminum hydroxide (found in some antacids);
  • Calcium supplements;
  • Colestid and cholestyramine (absorbs bile);
  • Iron supplements;
  • Magnesium supplements;
  • Raloxifene (an osteoporosis treatment);
  • Sucralfate (for ulcers);
  • Soy-based foods.

If you need to take any of the medications above, you should take them three or four hours before or after you take T4. A doctor will recommend the best time to take T4 (common times to take the medication are first thing in the morning or right before you go to bed when you have an empty stomach).

Within two weeks of taking synthetic T4 hormone supplements, you’ll begin to feel the effects of the treatment. When taken as directed — and with the right dose — you’ll find that T4 hormone replacement therapy can effectively manage your hypothyroid symptoms.

Ask any additional questions you may have from our medical experts, apply for free advice here.