Thyroid panel: TSH, free T4 and T3, what each one measures and why «normal» is not always normal

The thyroid panel is one of the tests that leaves the most doubts when you read the result: a high TSH with a free T4 marked as normal, or a TSH that landed right on the limit, and a lab note that explains nothing. It is a blood test that measures how the pituitary and the thyroid talk to each other, which is why a single number is rarely interpreted on its own.^[1] This text explains what each part measures (TSH, free T4, T3 and, when needed, antibodies), which combinations doctors usually describe, what moves TSH without the thyroid being ill, why «normal» depends on the lab and on age, and why the guidelines ask to repeat the test before drawing conclusions.

What each part of the thyroid panel measures

The thyroid produces two hormones, thyroxine (T4) and triiodothyronine (T3), and the pituitary directs it with a third, thyroid-stimulating hormone or TSH. The system works like a thermostat: if the pituitary sees little T4 in the blood, it produces more TSH so the thyroid works harder; once T4 goes above a certain level, TSH production is shut off.^[1] That is why TSH moves in the opposite direction to the thyroid, and why it is the test you start with: the American Thyroid Association considers it the best initial way to test thyroid function,^[1] and its hyperthyroidism guideline describes it as the single test with the highest sensitivity and specificity.^[4]

TSH: the pituitary signal

A high TSH indicates that the thyroid is not producing enough hormone, which is called primary hypothyroidism; a low TSH usually indicates that it produces too much, that is, hyperthyroidism.^[1] That is as far as TSH goes: it does not say what the cause is, and it can be high or low even when the thyroid is healthy.^[2] In some uncommon cases, an abnormal result reflects a pituitary problem rather than a thyroid one.^[2] So the rule is simple: if TSH comes out of range, the usual step is to order at least one more test, T4, T3 or antibodies, to understand what is going on.^[2]

Free T4 and total T4: the available hormone

T4 is the main form of thyroid hormone circulating in the blood, and most of it travels bound to carrier proteins. Total T4 measures the bound and the free hormone together, and changes when those proteins change; free T4 measures only the part that is not bound and can enter the tissues and act.^[1] The difference matters: the estrogens in oral contraceptives and in pregnancy raise the carrier proteins and, with them, total T4 and T3, without the thyroid having changed anything. That is why the recommendation is to ask for TSH and free T4 together, which in those cases usually stay within the range.^[1]

The two classic combinations come from here: an elevated TSH with a low free T4 indicates primary hypothyroidism, due to disease in the thyroid itself; a low TSH with an elevated free T4 is found in hyperthyroidism.^[1] Everything in between those two extremes, which is most of the results that raise doubts, is discussed in the table below.

T3: useful mostly when TSH is low

T3 helps confirm hyperthyroidism and estimate its severity; by contrast, it rarely adds anything in hypothyroidism, because it is the last hormone to become abnormal: a person can have a very high TSH and a low free T4 and still a normal T3.^[1] Free T3 can be measured, but it is often not reliable and therefore not usually helpful.^[1] The hyperthyroidism guideline sums it up: when suspicion is strong, measuring TSH, free T4 and total T3 in the same sample improves the accuracy of the first step.^[4]

Anti-TPO antibodies: the next step, not the first

A basic thyroid panel does not usually include antibodies, and that is normal. The two most common are antibodies against thyroid peroxidase (anti-TPO) and against thyroglobulin; when they are positive in someone with hypothyroidism, they point to Hashimoto thyroiditis, an autoimmune disease.^[1] Hashimoto is the most frequent cause of hypothyroidism.^[7] The Mexican IMSS guideline considers them useful to confirm that the origin is autoimmune and to estimate the probability that a high TSH with normal free T4 will progress to overt hypothyroidism, but does not recommend ordering them routinely once hypothyroidism is established.^[6] In hyperthyroidism the antibody that matters is a different one, the one acting on the TSH receptor that is behind Graves disease.^[1]

Combinations of TSH and free T4 and what the doctor usually orders next

This table gives no diagnoses: it describes the patterns a doctor recognizes when seeing TSH next to free T4, and what they usually order next according to the guidelines.^[1][4][5][6] If your result has only one of the two, the first thing missing is the other.

Two comments on the table. First: a low TSH with a low free T4 is an uncommon pattern that can point to the pituitary or to a recent illness; the doctor usually repeats once the person has recovered.^[1][2] Second: the row «both normal with symptoms» is the most frequent in practice. In most healthy people a normal TSH means the thyroid is working properly,^[1] and the symptoms that lead to ordering the panel, tiredness, cold, dry skin, hair loss, brain fog, are very varied and subjective, in the words of the Mexican guideline;^[6] a diagnosis of hypothyroidism is never based on them alone.^[7] If your TSH is in range and the symptoms continue, these texts explore other directions: tired all the time, hair loss and brain fog.

What moves TSH without the thyroid being ill

TSH is sensitive, and that is a virtue and a problem at once: it reacts to things that are not thyroid disease. This is the list worth reviewing before worrying about an out-of-range value.^[1][2][6][7]

On medicines it is worth being concrete without giving names. The Mexican guideline lists drugs that alter thyroid function, among them one for heart rhythm and one for mood, and adds causes of high TSH that are not subclinical hypothyroidism: obesity, unmanaged adrenal insufficiency and kidney dysfunction; among the transient ones, the recovery phase of an illness and painless or postpartum thyroiditis.^[6] The NIDDK notes that certain heart medicines, bipolar disorder medicines and cancer medicines can lead to hypothyroidism,^[7] and the hyperthyroidism guideline describes that a heart rhythm medicine with a high iodine content causes a destructive thyroiditis in 5 to 10 % of those who take it.^[4] None of them is stopped because of a TSH: the list goes to the doctor and the decision is made there. It is the underlying reason why an isolated result says little: TSH measures the pituitary response to everything happening in the body, not only to the thyroid.^[2]

The TSH reference range is not a single one

If you compare your result with that of a relative who went to another lab, the ranges will probably not match, and it is not a mistake. The 2014 US hypothyroidism guideline takes 0.4 to 4.0 mIU/L as the reference range and as the target in those already receiving hormone;^[3] the 2013 European guideline puts the start of the mild subclinical zone at 4.0;^[5] the Mexican IMSS guideline classifies a TSH of 4.5 to 10 mIU/L as mild.^[6] Three serious documents, three different upper limits between 4.0 and 4.5, because each rests on different populations and laboratory methods. The range that counts for reading your result is the one printed by the lab that issued it, and even that range describes a population, not you: in this text on the reference range we explain why the most useful comparison is with your own previous results.

TSH and age

The Mexican guideline says it plainly: during aging TSH concentrations increase without that indicating thyroid pathology.^[6] The European and US guidelines agree on the practical consequence: age-specific, local reference ranges should be considered when interpreting the TSH of an older person.^[3][5] The European one goes further and proposes, for people over 80 to 85 with a TSH of up to 10 mIU/L, a wait-and-see attitude rather than intervening.^[5] In other words: a TSH of 5 at age 78 and the same TSH at 35 do not tell the same story. At the same time, hypothyroidism is more common after 60 and much more frequent in women than in men,^[7] so age rules nothing out; it only changes where the line is.

TSH in pregnancy

In pregnancy the cut-offs are different and stricter. TSH is often a little low during the first three months,^[2] and the Mexican guideline defines subclinical hypothyroidism of pregnancy as a TSH above 2.5 mIU/L with positive anti-TPO, and considers any TSH of 10 or more hypothyroidism regardless of free T4; targets change by trimester.^[6] The reason for so much precision is that unmanaged hypothyroidism during pregnancy can affect both mother and baby, and many women already taking thyroid hormone need a higher dose when they become pregnant, so the NIDDK asks to contact the doctor right away.^[7] A thyroid alteration with a pregnancy under way or planned is not left for the next check-up.

The subclinical zone: why the guidelines ask to repeat before concluding

The pattern that generates the most consultations is a high TSH with normal free T4. The Mexican guideline defines it this way, subclinical hypothyroidism is an elevated TSH with normal free T4, and divides it into mild, 4.5 to 10 mIU/L, and severe, above 10.^[6] The European one uses the same two grades, mild and severe, with the lower cut at 4.0.^[5] That border at 10 is what most changes the conversation with the doctor, because above it the guidelines weigh what to do differently.^[5][6]

What almost nobody says when handing over the result is that the guidelines conclude nothing from a single sample. The European one asks, for an initially raised TSH with free T4 in range, to repeat the measurement of TSH and free T4 together with anti-TPO antibodies, preferably after an interval of 2 to 3 months.^[5] The reason is in the table above: diurnal variation, recovery from an illness and passing thyroiditis raise TSH temporarily,^[6] and a share of the values that come out high the first time return to the range without anyone doing anything. Repeating is not letting it go: it is the way to know whether the result is yours or the circumstances’. And if, after confirming it, the doctor decides to observe without medicine, the Mexican guideline proposes checking TSH, free T4 and anti-TPO at six months.^[6]

The opposite side exists too. Subclinical hyperthyroidism is a low or undetectable TSH with T3 and free T4 within the normal range.^[4] It does not usually give marked symptoms, but the hyperthyroidism guideline reminds that unattended thyrotoxicosis is associated with atrial fibrillation, embolic events, muscle weakness, tremor and neuropsychiatric symptoms, and that cardiac evaluation may be necessary especially in the older patient.^[4] So, with a low TSH, the home data point that matters most is the resting pulse and its regularity: here we explain how to measure it and what is normal by age.

What to do with the result and when to repeat

With the result in hand, the useful thing is to turn it into a data point that can be compared, not into a conclusion. This is the sequence that follows from the guidelines:

1. Save the complete result, with units, with the lab range and with the conditions: time of the draw, whether you were taking biotin, whether you were ill or recovering, which medicines you were taking.^[1][6]
2. If free T4 is missing, ask for it together with TSH at the next draw: TSH alone is not enough to explain an out-of-range value.^[2]
3. High TSH with normal free T4: repeat TSH and free T4 in 2 to 3 months, under the same conditions, and ask your doctor about anti-TPO in that same sample.^[5]
4. Low TSH: book an appointment in the coming weeks; the doctor will decide whether to add total T3 and antibodies, and in the meantime record your resting pulse every morning.^[4]
5. No fasting is needed for TSH itself; if glucose or lipids are on the same order, the fasting is for them. Do not stop any medicine unless your doctor says so.^[2]
6. While you wait for the repeat, record weight and resting pulse once a week and note symptoms with dates. If your weight moves without you changing anything, this text on not losing weight explains what else is worth measuring.

Warning signs

Almost everything around a thyroid panel allows waiting two or three months and repeating. There are exceptions, and it is worth having them clear so as not to confuse prudence with delay:

• Fever, confusion or decreased alertness, a very fast or irregular pulse in someone with hyperthyroidism or with symptoms of an overactive thyroid: it may be a thyroid crisis, a sudden worsening that sometimes follows an infection or strong stress, and it is treated in hospital. With a change in consciousness, dizziness or a fast, irregular heartbeat, go to the emergency room.^[8]
• Extreme drowsiness, below-normal body temperature, slow breathing, unresponsiveness: these are the signs of the most severe form of hypothyroidism, myxedema coma, a medical emergency treated in hospital.^[9]
• A lump in the neck that grows, difficulty swallowing or breathing: thyroid nodules are very common, by age 60 about half of all people have one and more than 90 % are benign, but a large nodule can press on the windpipe or the esophagus, and one that keeps growing needs assessment without waiting for the next panel.^[10]
• A pregnancy under way or planned with any thyroid alteration: consult without waiting, because the cut-offs are different, the hormone dose usually changes and unmanaged hypothyroidism can affect mother and baby.^[6][7]
• Palpitations with an irregular pulse that do not settle, even if the TSH is not yet confirmed: atrial fibrillation is among the complications of an overactive thyroid, and it is checked first.^[4][8]

What to bring to the doctor

A thyroid appointment is decided in a few minutes, and it almost always turns on two questions: does this repeat? and what was around it? Arriving with the answers changes the quality of the decision.

1. The complete result, with units and the lab range, and every previous panel you have, even from another lab: the trend is worth more than the number.
2. The conditions of each draw: time, recent illness, biotin, pregnancy or recent birth.^[1][2][6]
3. The full list of medicines and supplements, with doses, including over-the-counter ones and multivitamins with biotin or iodine.^[1][6]
4. Symptoms with dates: when the tiredness, the cold, the dry skin, the hair loss, the palpitations or the weight change started, and how many days a week they appear.^[2]
5. Weight and resting pulse from the last weeks, with dates; if you wear a watch, the night-time pulse helps too.^[4]
6. History: relatives with thyroid disease, a birth in the last six months, radiation to the neck or chest, other autoimmune diseases.^[7]