Thyroid Function Tests: What is Normal?

Thyroid Function Tests

Has this happened to you? You go to the doctor with one or more of the following complaints all of which are symptoms of low thyroid or hypothyroidism, but your thyroid function tests come back normal.  Those symptoms include:

• feeling fatigued
• feeling down
• having memory issues or brain fog
• feeling cold
• skin is dry
• hair is thinning
• eyebrows are thinning
• experiencing constipation
• weight gain
• face is puffy
• cholesterol is elevated
• experiencing muscles aches or stiffness.

Your doctor orders thyroid function tests and then calls you and says, “Good news. Your thyroid levels are normal.” You ask, “Well, what’s wrong with me?” To which there is no answer, or you might be told these symptoms simply occur with aging, or that you might be depressed. Don’t you love to hear that?

Next thing you know you are placed on an antidepressant to help the fatigue and down moods, an anti-inflammatory to help with the muscle aches, a medication to improve your bowel habits, and a cholesterol lowering drug. Plus, you are told to exercise more and make better food choices to lose weight. But no solution is offered for your dry skin or thinning hair and you are referred to a dermatologist.

What’s challenging for doctors though is most people do not come to the office with such a classic presentation. Most of the time they have just one or more symptoms of low thyroid initially with the most common ones being fatigued, feeling down some, and gaining weight. So the diagnosis of low thyroid is frequently delayed if not totally overlooked.

But, the real problem is relying solely on thyroid function tests to make a diagnosis. Some medical problems are a clinical diagnosis. In other words, the diagnosis is made entirely based on symptoms and exam findings like sinusitis. Other problems are largely laboratory diagnoses like elevated cholesterol.

Problems related to the endocrine system like the thyroid gland are largely viewed as laboratory diagnoses in conventional medicine for reasons which are entirely unclear. As physicians we are supposed to treat patients not diagnostic tests. Plus, no diagnostic test is perfect. And, the reference ranges established for most labs are arbitrary and not based on science or medical evidence.

You may have all the classic symptoms of low thyroid, or low testosterone, or low estrogen, but if your lab tests fall within the reference range – mistakenly called the normal range – many doctors are not going to treat you.

And, that makes little sense especially if other explanations for your symptoms have been excluded. Afterall, what is the harm in treating someone for low thyroid on a trial basis? Minimal if properly monitored is the answer.

Statistically it is more likely that a single medical problem is causing several symptoms that manifest within a short period of time than several medical problems all occurring at once to explain many symptoms. Does that make sense?

In other words, a physician is going to be right more often trying explain a host of symptoms occurring within a short time span with as few diagnoses as possible – usually one in most cases – than he or she is making several diagnoses to explain several symptoms.

Deficiency in any hormone is associated with a host of symptoms and the thyroid is no different. So when many symptoms occur within a short period a hormone deficiency should be considered.

So what exactly does the thyroid do?

The Function of the Thyroid Gland

The thyroid gland is a butterfly shaped gland that sits in front of the larynx or windpipe. Functions of the thyroid gland include:

• assists in cell growth and differentiation.
• regulates metabolism in each cell of the body.
• regulates temperature.
• lowers cholesterol and helps breakdown fats.
• increases brain metabolism and protects against cognitive decline.

So how is thyroid function measured? There are three ways to assess thyroid function.

• Clinical symptoms and signs
• Laboratory tests (blood work)
• Basal body temperature

Clinical signs and symptoms

Clinical signs and symptoms can be used like the ones discussed above to gauge how well your thyroid gland is working.  The symptoms above are the most common, but there are over 200 symptoms attributable to low thyroid.

Laboratory tests

Before we go into that you need to understand how reference ranges are established. Surprisingly they have little to do with what a healthy level is – in this case a healthy thyroid level. Most reference ranges are established by going out two standard deviations from the mean value in a population creating the typical bell-shaped curve.

Two standard deviations from the mean in both directions encompasses 95% of the population. So right out of the gate 95% automatically have a  test value that is considered “normal.”

On either end of this middle 95% are 2.5% on each end who have lab values that are either low or high. This is far from a scientific way to determine normal which makes practicing evidence-based medicine challenging when the reference ranges are not based on science or evidence.

Not all the reference ranges for lab tests are arbitrarily set like this, but this methodology does apply to hormone levels of any kind. Yet, some thyroid experts estimate that as many as 40% of the population suffer from symptoms related to low thyroid.

The reference range of TSH, T4, and T3 vary from lab to lab as each establishes its own reference ranges for its lab results based on the population it serves. This is important to recognize and also highlights another shortcoming of relying solely on lab results to diagnose thyroid disease.

For instance, in Michigan and other Midwestern states many suffer from iodine deficiency. Having a large population low in iodine skews or shifts the reference range for T4 and T3 to the left or low-end of the spectrum. So someone from Michigan who has thyroid studies in the middle of the range for Michigan will likely be in the low-end of the range when compared to someone living elsewhere. But, compared to their peers in Michigan they are “normal” even though they may be suffering from low thyroid.

Below are the typical reference ranges but the range in your lab may be different.

• TSH          0.2 to 5.0 mU/L with optimal being less than 2.5 mU/L
• Free T4    0.8 to 1.8 mcg/dl with optimal being greater than 1.3 mcg/dl
• Free T3    2.3 to 4.2 pg/dl with optimal being greater than 3.2 pg/dl

Some labs report their reference ranges using different units of measure. Optimal, though, is the upper half of the range for free T4 and T3 and the lower half of the range for TSH.

Basal body temperature

The body functions best within a narrow range of body temperature that ranges from 97.8ºF to 99.8ºF. Outside that range bodily processes and enzymatic reactions are hampered. Normal body temperature is 97.8ºF to 98.2ºF when measured under the arm. Add a degree for body temperature measured rectally or orally.  Regardless of race, gender, and age body temperature should be normal as we just defined.

We all hear of people who say they always run a low body temperature. That’s not normal. Low thyroid function should be considered in those individuals. Some thyroid experts monitor body temperature to guide thyroid replacement treatment more so than they do blood levels of thyroid hormones.

They will start patients on low dose thyroid replacement and increase the dose slowly until the patient’s body temperature approaches the normal body temperature range. That’s how reliable and important body temperature is. Body temperature should be measured first thing in the morning before getting out of bed.

Thyroid Function Tests

The main thyroid function tests include;

• TSH
• free T4
• free T3

Other tests that can be done include:

• reverse T3
• thyroglobulin antibodies
• antithyroid peroxidase antibodies

We are going to focus on TSH, T3, and T4 only. TSH is produced by the anterior pituitary located in the brain. TSH then stimulates the thyroid gland to produce thyroid hormones. This includes T4 and T3 predominantly and minutes amount of T1 and T2. T4 contains four iodine molecules and T3 contains three iodine molecules and so on.

There is a 4:1 ratio of T4 to T3, but T3 is four times more potent in terms of its effects on the cells. In addition, T4 is converted into T3 in the cell. So in many regards T4 is actually a pro-hormone and T3 is the active thyroid hormone. The conversion of T4 to T3 is hampered by certain medications, chronic disease, and aging.

Like other hormones thyroid regulation is governed by a negative feedback loop. When T4 and T3 levels are adequate the anterior pituitary shuts off production of TSH which then shuts off production of T4 and T3 by the thyroid gland. When T4 and T3 are low TSH production kicks in and more T4 and T3 is produced.

It’s a little bit more complicated than that but that is the gist of how thyroid hormones are produce. In actuality, the hypothalamus produces thyroid releasing hormone or TRH which stimulates TSH. So when T4 and T3 are low TRH is released stimulating the pituitary to produce TSH.

Many physicians only measure a TSH to determine thyroid function. If TSH is high then it is assumed that T3 and T4 are low. And, if TSH is “normal” then it is felt there must be adequate levels of T4 and T3.

But, why not measure T4 and T3? If we want to know if estradiol level is low we measure it and not rely on FSH which stimulates ovarian function.  And, if we want to know if testosterone is low we measure it and not rely on levels of LH which stimulate testosterone production.

But, physicians for the most part are taught that all that is needed is a TSH level, and if it is high then get a T4 level but not a T3 level. And, only treat for low thyroid if TSH is high and T4 is low. It makes little sense since T3 activity is what you really want to know.

Also, many physicians only prescribe a T4 containing replacement medication like Synthroid with the thought that T4 will convert to T3 if the body really needs T3, but the conversion of T4 to T3 is not reliable in face of chronic disease and medications. We believe that most patients are likely to benefit more from thyroid replacement if T4 and T3 are prescribed in the 4 to 1 ratio that they are naturally made by the body.

As a rule of thumb our practice is to treat patients who have symptoms of low thyroid (unexplained by other diagnoses) and T4 and/or T3 levels in the lower half of the reference range and TSH levels in the upper half of the range. We do so on a trial basis – typically three months – checking thyroid function tests periodically. If the patient’s symptoms do not improve then we stop treatment.

One more comment about thyroid function tests. To really monitor blood levels and compare apples to apples it is essential that blood tests be done the same time each lab draw and if the patient is on thyroid replacement that they take the medication the same time everyday. Otherwise it becomes a bit of a guessing game trying to interpret results. That’s one advantage of using body temperature – it is much easier to interpret.

What Blocks the Conversion of T4 to T3?

There are medications, nutritional deficiencies, foods, toxins, and disease that affect the conversion of T4 toT3 making relying on TSH and T4 levels to diagnose and treat low thyroid problematic.

The following drugs affect T4 to T3 conversion:

• beta blockers – used for high blood pressure and heart disease.
• birth control pills.
• estrogen making thyroid monitoring essential in women receiving hormone replacement therapy.
• lithium used in medications to treat bipolar disorder.
• some radiologic contrast dyes.
• phenytoin used to treat seizure disorders.
• corticosteroids use to treat a variety of inflammatory conditions.
• theophylline used to treat asthmatics.

Nutritional deficiencies include:

• vitamin A.
• vitamin B2, B6, and B12.
• chromium.
• copper.
• iodine.
• iron.
• selenium.
• zinc.

Foods include:

• cruciferous vegetables
• soy

Toxins and chemicals include:

• alcohol.
• fluoride.
• lead.
• mercury.
• pesticides.
• radiation.

Diseases and medical conditions include:

• aging.
• diabetes.
• stress.
• surgery.

What is a normal thyroid function?

Well we probably should say “optimal” thyroid function. It really varies from person to person. But, it is the level or range in which you do not experience symptoms of either low or high thyroid (T3 and T4).

We think is it wise to have a complete hormonal panel performed between ages 30 to 35 when most individuals are feeling good and healthy in order to establish what is normal or optimal for them providing a baseline for future use when their hormone levels eventually decline. You do not have to see a physician to do this. There are plenty of lab companies that can be found online that you can go to directly to have such hormone panels performed.

In a future article we will discuss natural ways to improve you thryoid function.