High TSH: what it means and how to read a thyroid test

In 1891, the British physician George Murray published a report that seemed bold at the time. He injected an extract of sheep thyroid into a woman with myxedema, the name then given to the condition caused by a shortage of thyroid hormone. Her symptoms improved. It was one of the first times medicine managed to replace a hormone the body had stopped producing.

Murray had no lab test to tell him how much hormone was missing. The diagnosis came from looking at the patient. Thyroxine, the hormone we now call T4, was not isolated until 1914, by Edward Kendall, and newborn screening for congenital hypothyroidism began to be described in the 1970s, in the work of Jean Dussault.

Today, what Murray had to guess at fits in a single tube of blood. The most commonly ordered thyroid test measures TSH and, often, free T4. It is common to open a lab report on your phone, see "high TSH" and feel alarmed. This article explains what those numbers mean, without taking the place of the person who interprets the test: your doctor.

What does the thyroid do in the body?

The thyroid is a small, butterfly-shaped gland at the front of the neck, just below the Adam's apple. It makes two main hormones, T4 (thyroxine) and T3 (triiodothyronine), which help control how the body uses energy.

According to the Brazilian Society of Endocrinology and Metabolism (SBEM), the thyroid influences growth in children and teenagers, weight, memory, concentration, mood, menstrual cycles and fertility. With too little hormone, many functions slow down. With too much, they speed up.

What is TSH, and why does it go up?

TSH stands for thyroid-stimulating hormone, also called thyrotropin. It is not made by the thyroid but by the pituitary gland, a gland at the base of the brain. TSH tells the thyroid how much hormone it needs to produce.

The system works like a thermostat. If the level of thyroid hormone in the blood is low, the pituitary makes more TSH, like someone turning up the volume to be heard. If the hormone level is high, the pituitary turns TSH down.

That is why the logic looks backward at first:

• High TSH usually means the thyroid is making too little hormone (hypothyroidism).
• Low TSH usually means there is too much hormone in circulation (hyperthyroidism).

MedlinePlus, the service of the U.S. National Library of Medicine, stresses that TSH shows whether the thyroid is making too much or too little hormone, but it does not explain the cause. For that, the doctor usually orders other tests.

What does free T4 show?

T4 is the main hormone the thyroid produces. In the blood it comes in two forms. Most of it is bound to proteins, like a reserve. A small portion circulates unbound, and that is the part that enters the tissues and does the work: free T4.

A lab can measure total T4 (bound and unbound together) or only free T4. According to MedlinePlus, free T4 is considered more accurate and is therefore used more often. Even so, T4 alone does not settle a diagnosis. It is read alongside TSH, because each helps explain the other. MedlinePlus notes that a low free T4 without a raised TSH can, more rarely, point to a problem in the pituitary, which is why the two are usually ordered together.

The most common combinations are:

• High TSH with low free T4: a pattern that usually appears in hypothyroidism.
• Low TSH with high free T4 (and sometimes high T3): a pattern that usually appears in hyperthyroidism, according to the NHS, the United Kingdom's public health service.
• Slightly abnormal TSH with normal free T4: a situation that calls for a recheck and more context. The doctor may follow it with repeat tests.

What is a thyroid test for?

A TSH test shows whether the thyroid is working properly. A doctor may order it when there are symptoms of too much or too little hormone, when thyroid disease runs in the family (it tends to repeat among relatives), when a lump or swelling appears in the neck, or to monitor someone who is already in treatment.

TSH is also the test used on babies shortly after birth. In Brazil, the heel-prick test (teste do pezinho) screens for congenital hypothyroidism, and SBEM advises doing it, ideally, between the third and fifth day of life.

Which symptoms can lead to a thyroid test?

For too little hormone (hypothyroidism), the NHS and MedlinePlus list extreme tiredness, feeling cold, weight gain, constipation, trouble concentrating, low mood, dry skin and hair, hair loss, a hoarse voice, and irregular or heavier periods. These symptoms tend to appear slowly, and many people take months or years to notice them.

For too much hormone (hyperthyroidism), the more frequent signs are weight loss despite eating more, a fast or irregular heartbeat, nervousness, irritability, trouble sleeping, trembling hands, heavy sweating, heat intolerance and loose stools. According to the NHS, there can also be a swelling in the neck from an enlarged thyroid (goiter) and eye problems such as redness and dryness.

Who is more likely to develop hypothyroidism?

Anyone can. According to the NHS, Hashimoto's disease is more common in women and usually appears between the ages of 30 and 50. The chances are also higher in people with relatives who have thyroid disease, in those who have had neck surgery or radiation or treatment for hyperthyroidism, and with the use of certain medications. A lack of iodine in the diet is also a possible cause of hypothyroidism. That is why, at the appointment, the doctor usually asks about family history, recent pregnancy and other autoimmune diseases.

Do you need to prepare for the test?

It is an ordinary blood draw from a vein in the arm, and it takes a few minutes. According to MedlinePlus, the TSH test itself usually needs no special preparation. But a few precautions apply:

• If other tests were ordered with it, you may need to fast for a few hours. Follow the lab's instructions.
• Some medications and supplements can interfere with the result. Tell your doctor everything you take, but not stop any medication on your own.
• High doses of biotin (vitamin B7), common in hair and nail supplements, can distort thyroid tests. A study published in the journal Thyroid showed that, with one lab method, TSH came out falsely low and free T4 falsely high. Before the blood draw, ask the lab and your doctor whether you need to pause the supplement for a few days.
• If you take levothyroxine, follow your doctor's guidance on when to take the dose on the day of the test.

How can you read the result without panicking?

On the report, each value appears next to a reference range. It is tempting to compare the number with the range and draw your own conclusion, but there are three reasons to take it slowly.

First, reference values vary from lab to lab, depending on the method. The NHS explains that what counts as normal depends on factors such as age and the testing technique. That is why it makes no sense to compare your number with a chart you found online.

Second, TSH can come back abnormal even when the thyroid is healthy. MedlinePlus gives a few examples:

• serious illnesses unrelated to the thyroid can lower TSH for a while;
• TSH can be somewhat higher in people over 80 without it meaning disease, and the doctor judges each case individually;
• pregnancy also changes TSH, which tends to be slightly lower in the first three months.

Third, a single result rarely settles the question. When TSH is abnormal, the doctor may repeat the test and order free T4, T3 and other blood tests to find the cause. According to the NHS, Hashimoto's disease, an autoimmune condition in which the body's defenses attack the thyroid, is the main cause of hypothyroidism in the United Kingdom, and Graves' disease is a common cause of hyperthyroidism.

The result is an important clue, but it is the doctor who puts the clues together with your history and a physical exam.

Is a high TSH dangerous?

Most of the time, hypothyroidism develops slowly and is not an emergency. The NHS says that with proper follow-up and treatment, people usually live a normal life, with regular blood tests to adjust their care.

Left untreated or poorly controlled, the shortage of hormone can cause complications. The NHS lists heart and bone problems and complications in pregnancy, and SBEM notes that it can also raise cholesterol. Anyone with hypothyroidism who is pregnant or planning a pregnancy should tell their doctor, because follow-up usually changes during that time.

When to see a doctor

See a doctor if you have symptoms that suggest too little or too much thyroid hormone, if you notice a lump or swelling in your neck, if thyroid disease runs in your family or if you received an abnormal result. A general practitioner can order the first tests and, if needed, refer you to an endocrinologist, the specialist in hormones. According to SBEM, a large share of the population develops thyroid nodules at some point in life and most are benign, but every nodule that is found should be examined by a doctor.

Go to an emergency room or call SAMU, Brazil's emergency medical line, at 192 if there is chest pain, shortness of breath, a racing heart or fainting, or if someone, especially an older person, becomes confused or very drowsy and hard to wake. The NHS notes that, in very rare cases, a severe shortage of hormone can lead to an emergency called myxedema coma, which requires hospital treatment.

From sheep extract to a tube of blood

George Murray's patient depended on the doctor's eye and a good measure of luck. Today, a few milliliters of blood show how the conversation between the pituitary and the thyroid is going, in an advance that starts with the heel-prick test and stays with a person throughout life.

The takeaway is balance: a high TSH on your report deserves attention, not panic. Take the result to your doctor, describe your symptoms and the medications you take, and let the doctor read the full picture. The numbers only make sense within your own history.

Frequently asked questions

Does a high TSH always mean hypothyroidism?

No. It usually suggests that the thyroid is making too little hormone, but it can vary with age, pregnancy, other illnesses and some medications. The doctor confirms it with free T4 and, sometimes, by repeating the test.

Do I need to fast for a thyroid test?

TSH generally does not require fasting, but other tests ordered with it may. Follow the guidance of the lab and your doctor.

Which doctor treats the thyroid?

A general practitioner can start the workup, and the endocrinologist is the specialist in thyroid diseases.

Can a supplement change a thyroid test?

Some can, especially those with high-dose biotin. Ask the lab and your doctor before the blood draw about pausing the supplement for a few days, and do not stop anything on your own.

Sources

• MedlinePlus (U.S. National Library of Medicine). TSH (Thyroid-stimulating hormone) Test. medlineplus.gov
• MedlinePlus (U.S. National Library of Medicine). Thyroxine (T4) Test. medlineplus.gov
• NHS. Underactive thyroid (hypothyroidism). nhs.uk
• NHS. Overactive thyroid (hyperthyroidism): Diagnosis. nhs.uk
• NHS. Overactive thyroid (hyperthyroidism): Symptoms. nhs.uk
• Sociedade Brasileira de Endocrinologia e Metabologia (SBEM, Brazilian Society of Endocrinology and Metabolism). 10 coisas que você precisa saber sobre tireoide. endocrino.org.br
• Ylli D et al. Biotin Interference in Assays for Thyroid Hormones, Thyrotropin and Thyroglobulin. Thyroid, 2021. PMC
• Connelly KJ, Park JJ, LaFranchi SH. History of the Thyroid. Hormone Research in Paediatrics, 2022. PubMed

Text by Rai, RaisLife's virtual reporter (artificial intelligence), with automated editorial review.