Thyroid blood tests are generally straightforward and accurate, but certain factors can affect your individual results. Persistent fluctuations can occur, and they are a sign that you need your medication dose changed. But your test results can also be falsely skewed due to factors such as the time of day, or temporary conditions, such as illness or pregnancy—meaning your recorded levels may not reflect a true change in your thyroid condition.

Minor alterations due to factors unrelated to your thyroid disease can make mild disease less noticeable or may make your thyroid condition seem worse than it actually is. It's a good idea to be aware of these factors so that you will have the most precise thyroid test results.

​Illustration by Emily Roberts, Verywell

Fasting

Studies have shown that early morning thyroid blood tests taken after overnight fasting translate to higher TSH levels compared to those taken later in the day with no fasting.﻿﻿

This fasting/non-fasting variation can be especially problematic if you have subclinical hypothyroidism, defined by a mildly elevated TSH and normal free T4. The diagnosis and monitoring of this condition relies solely on the TSH value since the T4 and T3 levels are typically normal.

A diagnosis of subclinical hypothyroidism may be missed if you get your blood test when your TSH value is at its lowest of the day due to a non-fasting afternoon blood draw.

Lifestyle

Other factors, such as stress, sleep deprivation, dieting, and different times of the menstrual cycle may impact thyroid test result, although the evidence is unclear. The best way to avoid false fluctuations in lab test results is to have your thyroid levels checked under the same conditions each time.﻿﻿

If you are taking thyroid hormone replacement, it's probably sensible to schedule your TSH blood draws around the same time of day and in the same manner (fasting/non-fasting).

Medication Use

Certain medications can cause thyroid dysfunction by interfering with the body's thyroid hormone levels or with their action, potentially altering TSH as the body attempts to compensate for high or low thyroid hormone activity.

Thyroid Hormone Function

A few examples of medications that alter thyroid hormone function and test results include:﻿﻿

Lithium

Amiodarone

Iodine or kelp supplements

Immunomodulating drugs like interferon alpha and interleukin-2

Chemotherapeutic drugs like Lemtrada (alemtuzumab) ﻿ ﻿

﻿ Tyrosine kinase inhibitors, like Nexavar (sorafenib) or Sutent (sunitinib) ﻿ ﻿

Thyroid Hormone Absorption

Certain supplements and medications can also interfere with your digestive system's absorption of thyroid hormone replacement or antithyroid medications. Medications like calcium carbonate, iron sulfate, and the proton pump inhibitors Prilosec (omeprazole) and Prevacid (lansoprazole) may prevent you from getting enough of your medication into your system.﻿﻿

Thyroid Hormone Test Results

Other medications can interfere with thyroid laboratory measurements, but not with the actual functioning of the thyroid.﻿﻿

A few examples of these medications include:

Certain nonsteroidal anti-inflammatories (NSAIDs)

Certain anti-convulsants

Heparin (a blood thinner)

Lasix (furosemide)

Glucocorticoids

Isotretinoin

Lastly, research has found that the supplement biotin (taken in doses of five to 10 milligrams) can interfere with the measurement of thyroid blood tests. Therefore, it's recommended that people who take biotin stop doing so two days prior to having their thyroid blood tests.

Pregnancy

Due to a number of factors, thyroid hormone levels change during pregnancy. In fact, the normal reference ranges change throughout pregnancy. The standardized ranges may also differ based on the lab where you get your blood test.

The American Thyroid Association recommends that a doctor use trimester-specific reference ranges for TSH and free T4 ranges during pregnancy.

References ranges for thyroid blood tests done on non-pregnant individuals are as follows:﻿﻿

TSH 0.5-4.7 mIU/L

Free T4 (FT4) 8.0-18 pmol/L

Free T3 (FT3) 2.30-4.2 pmol/L

In comparison, here are the reference ranges for pregnant women.﻿﻿

First Trimester:

TSH 0.1 - 2.5 mIU/L

FT4 10.30-18.11 pmol/L

FT3 3.80-5.81 pmol/L

Second Trimester:

TSH 0.2 - 3.0 mIU/L

FT4 10.30-18.15 pmol/L

FT3 3.69-5.90 pmol/L

Third Trimester:

TSH 0.3 - 3.0 mIU/L

FT4 10.30-17.89 pmol/L

FT3 3.67-5.81 pmol/L

Illness

Several illnesses can affect thyroid hormone results temporarily. Diarrhea can interfere with your medication absorption and may alter your lab results. Sometimes, infections or a bout of an inflammatory condition such as lupus can also alter your results until the illness resolves.

A serious illness that is of a degree requiring hospitalization in an intensive care unit may temporarily affect thyroid function and thyroid test results.

This syndrome, called nonthyroidal illness or sick euthyroid syndrome, is characterized by a low TSH level with a low T4, free T4, and T3 level.﻿﻿ Treatment aimed at correcting the thyroid levels is not recommended, as these alterations in thyroid levels are thought to actually be protective during critical illness.

A Word From Verywell

In the end, it is important to be consistent about your thyroid tests and to try to take them under similar conditions each time. Keep in mind that if your thyroid blood tests seem "off" for no good reason, a repeat blood test is a reasonable next step.