Have you ever wondered why your thyroid lab tests are normal even when you’re experiencing low thyroid symptoms? Fatigue, heavy head, weight gain, lack of motivation, and low body temperature are all linked to thyroid function. If your thyroid labs don’t match up with how you feel, it may be related with daily fluctuations in hormone levels. Thyroid hormone levels vary throughout the day changing from morning to afternoon, ultimately affecting decisions about thyroid health.

Several things impact thyroid stimulating hormone (TSH) circulating levels and ultimately blood test results. Daily circadian rhythms cause TSH to fluctuate by as much as 0.95 mIU/mL to 2.0 mIU/mL throughout the day. TSH levels are highest in the early morning and decrease late afternoon to mid-evening. TSH levels even fluctuate on average 0.75 mIU/mL on a monthly basis in healthy adults. Age, race, other illnesses, and season changes impact TSH levels.

Scientists evaluated TSH levels in patients with hypothyroidism. Blood tests were completed between 8:00 - 9:00 AM and 2:00 - 4:00 PM. Average morning TSH levels were 5.83 mU/L and afternoon levels dropped to 3.79 mU/L. For those who had their blood drawn in the afternoon, it resulted in at least 50 percent of the patients not being diagnosed with hypothyroidism. Optimal functional TSH levels are 1.0-2.0 mU/L.

The cascade of thyroid hormone production with the hypothalamic-pituitary-thyroid (HPT) axis is regulated by the SCN, or master body clock in the brain which talks to the clocks within the thyroid gland. The master clock-circadian rhythm cycle depends primarily on light signals that enter our eyes. The endocrine system including thyroid hormone, cortisol, leptin, and insulin secretion is governed by circadian rhythms. TSH and triiodothyronine or (T3) peak at 1:00 -2:00 AM and 2:30 - 3:30 AM respectively with the rhythms directly tied to the SCN master body clock.

TSH and thyroid hormone levels change as a result of disruptions to the clock. Shift workers are more likely to have elevated TSH levels and are more susceptible to autoimmune thyroid problems due to circadian rhythm changes. Obesity, cancer, metabolic syndrome/diabetes and autoimmune disorders are linked with disrupted endocrine body clocks.

TSH signals are affected too by the adrenals, the HPA (hypothalamus-pituitary-adrenal) axis and its signaling hormone ACTH. ACTH is similar to TSH. The hypothalamus and pituitary send out signals to produce TSH and ACTH. TSH signals the thyroid gland. ACTH signals the adrenal glands for cortisol release. ACTH stim test assesses how well adrenal glands are able to mount a response to stress.

A February 2018 study showed that an ACTH stim test caused an average 17 percent decrease of TSH output in just one hour. Cortisol released in response to acute stress dampens or lowers TSH levels as a sort of protective-brake mechanism. So, if on the way to the blood draw appointment for TSH labs something upsetting and stressful happens, it may end up influencing your test results.

What does this mean?

1. Thyroid hormone levels vary throughout the day and with other factors. TSH is highest first thing in the morning.

2. Disrupted circadian rhythms and body clocks will impact thyroid hormone secretion.

3. Acute stress and the body’s response to it will cause TSH levels to decline in just a short amount of time.

Helpful Points

1. If you have thyroid labs routinely completed, have them taken at the same general time, for example between 8:00 - 9:00 AM. One lab test for TSH does not give the full picture of how the thyroid is functioning. Keep an eye on TSH levels every 2-3 months if you have significant concerns. Make sure free T4, free T3, reverse T3, and thyroid antibodies are measured.

2. Get your body rhythms back on track. Work on your sleep schedule and routine to get your body clocks in sync. Regular meal times, not snacking, and following The Leptin Diet helps entrain body clocks.

3. Try to keep the time before the lab draw stress-free. If you have a really stressful commute to the lab, then see if you can take some time to de-stress before the lab draw.

4. Protect your thyroid health against other challenges that interfere with thyroid function. Infections (Candida, H. pylori, Epstein Barr, etc.), electromagnetic exposure (EMF, wired and wireless), plastics, xenoestrogens, pesticides, herbicides, perchlorates (bisphenols, Roundup, jet fuel, petroleum products, etc.), and food allergens and intolerance (wheat/gluten, soy, goiter-producing foods) may affect thyroid hormone levels and function. High levels of stress and adrenal stress, gut inflammation, and generalized inflammation causes altered thyroid function and hormone output. Blood sugar, leptin, insulin, sex steroid hormones, and TSH receptor site function also impact thyroid health.

5. There are some key nutrients your body needs for healthy thyroid function, such as tyrosine and selenium. Be sure to nourish your thyroid nutritionally. Learn more on our Thyroid & Metabolism health topic page.

Patients have been telling their providers for years that their thyroid lab tests don’t reflect how poorly they feel. Your thyroid health is more than numbers and a fixed reference range on a lab test. Discernment, awareness, and thinking outside the “reference range” box are needed for understanding thyroid function.

TSH is meant to be a screening test rather than a diagnostic test as TSH levels are considered an imprecise thyroid marker. Even then, it is not a perfect test. If TSH levels are off, then clearly something is going on. If TSH levels are borderline or “normal”, it does not fully reflect the full picture. Yet, many practitioners still consider TSH testing the only test needed for thyroid function. Use this information to help optimize and take charge of your thyroid health! Thyroid function affects everything.