How to prepare for the «TSH» test: rules for taking it

Thyroid-stimulating hormone (TSH) is the most sensitive and most frequently ordered indicator of thyroid function. It is where doctors begin their workup for fatigue, weight fluctuations, palpitations or hair loss. However, TSH has a circadian rhythm and responds to acute illness, certain medications and even dietary supplements. Our editorial team has compiled the rules that help you obtain a result reflecting the true state of the gland rather than the random circumstances of the blood draw.

What TSH shows and why preparation matters

TSH is produced by the anterior lobe of the pituitary gland. It stimulates the thyroid to synthesize thyroxine (T4) and triiodothyronine (T3). When thyroid hormones run low, the pituitary increases TSH secretion; when they are too high, it suppresses it. This negative feedback mechanism is what makes TSH so informative: even small changes in free T4 cause considerably larger shifts in thyroid-stimulating hormone.

Because of this sensitivity, TSH is recommended as the first screening test in most clinical guidelines, including those of the American Thyroid Association (ATA). In many situations, as the review by Sheehan (2016) emphasizes, TSH alone is sufficient and often the only test needed for the initial assessment of gland function.

The flip side of this sensitivity is vulnerability to external factors. A circadian rhythm, a night shift, an acute infection, the intake of glucocorticoids or high doses of biotin can shift the value enough that a person ends up with a false «subclinical hypothyroidism» or, conversely, misses a real problem.

Deviations caused by improper preparation are usually small, but it is precisely in the «gray zone» near the limits of the reference interval that they most often change the doctor's decision. That is why the collection conditions should be standardized every time, especially when tests are compared over time.

Time of day and food intake

TSH secretion has a pronounced circadian rhythm: hormone levels rise in the evening, peak at night, gradually decline in the morning hours, and reach their minimum in the second half of the day. The amplitude of the fluctuations is individual, but it is enough to make a result obtained at 8:00 noticeably different from an evening one.

Practical conclusion: it is best to have TSH tested in the morning, roughly between 8:00 and 10:00, and to schedule repeat tests at the same time. People who work night shifts should inform their doctor, because a shift in the sleep schedule also changes the hormonal rhythm.

As for food: some studies show that after breakfast TSH may be somewhat lower than in the fasting state. The effect is small, but for comparability of results most laboratories recommend giving blood on an empty stomach or after a light breakfast without fatty food. Plain water may and even should be drunk.

Prolonged fasting or a severe calorie deficit, on the other hand, should not be undertaken deliberately before the test: an energy deficit changes the entire thyroid profile, especially T3, and against this background TSH is harder to interpret.

Medications and supplements that distort the result

The most insidious factor of recent years is biotin (vitamin B7). It is contained in high doses in «hair, skin and nails» supplements and some sports formulas. Many immunoassays use the streptavidin–biotin system, and an excess of biotin in the blood interferes with the reaction. A study by Li and colleagues (JAMA, 2017) showed that taking 10 mg of biotin per day produced clinically significant false results in a number of hormone tests.

For TSH, which is measured by a «sandwich» method, biotin usually produces a falsely low result, whereas for free T4 and T3 it produces a falsely high one. The overall picture can end up mimicking hyperthyroidism. In 2017 the FDA issued a separate warning about such interference. It is generally advised to stop taking biotin at least 2–3 days before the test, and with very high doses even longer, after coordinating this with a doctor.

Patients who already take levothyroxine often ask whether they should skip the morning tablet. For TSH, the time the tablet is taken on the day of the test is not fundamentally important, because the hormone responds to T4 levels over weeks. However, if free T4 is ordered together with TSH, it is better to take the tablet after the blood draw.

The self-administration of thyroid hormones as «fat burners» deserves separate mention. Exogenous T3 or T4 suppress TSH, and such a test no longer characterizes the gland's own function. Moreover, this is a dangerous practice with a risk of arrhythmias and loss of muscle and bone mass; the doctor should know about the intake of any hormones.

Training, stress and acute illness

A single moderate workout the day before does not substantially change TSH, but exhausting exertion, competitions or several consecutive days of hard work with sleep deprivation can affect the hormonal axis. The logical rule is not to train intensively on the day of the test and the day before, and also to get a good night's sleep.

Acute psychological and physical stress raises cortisol levels, which can suppress TSH secretion. That is why giving blood right after a night shift, a flight across time zones or a sleepless night is not the best idea.

During acute illnesses — influenza, pneumonia, after surgery — the so-called non-thyroidal illness syndrome develops. In the illness phase TSH may decrease, and in the recovery phase it may temporarily rise above normal. Such changes do not indicate thyroid disease, and guidelines advise assessing gland function after recovery, unless there are urgent indications.

• do not train intensively on the day of the test or the day before;
• sleep your usual number of hours and avoid night shifts before the test;
• postpone a scheduled test to 2–4 weeks after an acute infection, unless the doctor decides otherwise;
• before the blood draw, sit quietly for 10–15 minutes.

Pregnancy is a special case. In the first trimester TSH physiologically decreases under the influence of chorionic gonadotropin, so trimester-specific reference intervals are used for pregnant women in line with the ATA 2017 guidelines.

Repeat tests and interpretation

Guidelines do not recommend treating a single moderate TSH deviation, especially one without symptoms, as a definitive diagnosis. The test is usually repeated after a few weeks — together with free T4, and if needed also with antibodies to thyroid peroxidase.

For a correct comparison it is advisable to give blood at the same laboratory: different analyzers have their own reference intervals, and the difference between methods can be comparable to the difference between «normal» and a «mild deviation».

Age is also worth remembering. Population data from NHANES (Hollowell et al., 2002; Surks, Hollowell, 2007) showed that in older people the TSH distribution shifts upward. A doctor will assess a moderately elevated TSH in an 80-year-old and in a 30-year-old athlete differently.

After starting treatment or changing the levothyroxine dose, TSH is checked no sooner than 6–8 weeks later: that is how much time is needed for a new balance to be established between the pituitary and the gland. More frequent tests will only create a false impression of instability.

Editorial conclusions

TSH is a simple and extremely informative test, but its result depends on the collection conditions. Morning timing, a stable routine, the absence of acute illness and a pause in biotin intake are the four rules that eliminate most false deviations.

Be sure to tell your doctor about all medications and supplements, including hormonal and sports ones, as well as about night shifts or recent infections. This information often matters no less than the number on the form itself.

You should not try to «correct» a TSH result on your own with iodine supplements or hormones. A deviation requires a repeat test and consultation with an endocrinologist.

We also recommend reading our materials on preparing for the free T4 and free T3 tests, as well as the review on the effect of energy deficiency on athletes' hormones.

References

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2. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670–1751.
3. Sheehan MT. Biochemical testing of the thyroid: TSH is the best and, oftentimes, only test needed — a review for primary care. Clin Med Res. 2016;14(2):83–92.
4. Li D, Radulescu A, Shrestha RT, et al. Association of biotin ingestion with performance of hormone and nonhormone assays in healthy adults. JAMA. 2017;318(12):1150–1160.
5. Hollowell JG, Staehling NW, Flanders WD, et al. Serum TSH, T4, and thyroid antibodies in the United States population (1988 to 1994): National Health and Nutrition Examination Survey (NHANES III). J Clin Endocrinol Metab. 2002;87(2):489–499.
6. Surks MI, Hollowell JG. Age-specific distribution of serum thyrotropin and antithyroid antibodies in the US population: implications for the prevalence of subclinical hypothyroidism. J Clin Endocrinol Metab. 2007;92(12):4575–4582.
7. Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315–389.