Does it Matter What Time of Day You Book Your Thyroid Testing?

thyroid testing

Thyroid testing. If you’ve ever gone in to your doctor for a basic physical assessment, you’ve likely gotten bloodwork. TSH (Thyroid Stimulating Hormone) is likely to be a part of the basic blood panel, especially if you’ve presented with symptoms of fatigue, constipation, thinning hair, feeling cold, dry skin, easy bruising, and weight gain.  TSH is a great first-screening test for thyroid, and is traditionally the standard test for assessing thyroid function.

What does TSH tell you?

TSH is a test that can indicate whether your thyroid hormone output is sufficient. TSH is secreted from your anterior pituitary, a gland which sits in your brain, (basically behind your nose). The anterior pituitary is part of the HPA axis – Hypothalamic-Pituitary-Adrenal axis, and is an essential part of regulating hormones from a number of organs, including the adrenals, ovaries, testes, and thyroid.

TSH is one of the preliminary thyroid-supporting hormones, acting to stimulate your thyroid gland to produce adequate amounts of thyroid hormone, and thus support healthy cellular metabolic function, (how efficiently your body functions).

What other tests can be done for thyroid?

TSH is not the whole picture of thyroid function, and indeed for a deeper understanding of thyroid and whether it’s contributing to your symptoms, one would require:

  • Free T4
  • Free T3
  • Thyroid Peroxidase (TPO) antibodies

Other, more in-depth testing for thyroid can include:

  • Total T4
  • Thyroglobulin
  • Thyroglobulin antibodies
  • Reverse T3
  • Urine iodine test
  • Urine halides (iodine, bromine, fluoride)

The Canadian range for TSH is 0.20 – 4.00 mIU/L. As your TSH increases, your thyroid response generally decreases, often reflecting hypothyroidism (if greater than 4.00 mIU/L). The functional (optimal) range for TSH is well-known to be 0.5 -2.5 mIU/L.

Optimal Timing for TSH Testing

In order to truly know where your TSH lies, and whether the TSH (and thyroid function) is related to your current symptoms, being consistent with timing of testing is important.

TSH exhibits a circadian-type of rhythm, fluctuating throughout the day and night. The early morning hours, from 2-4 AM, are when the TSH is likely to be at its highest, while between 4-8 PM in the evening, TSH is at its lowest. Your thyroid will not fluctuate outside of your physiologic range (whatever it may be for you in your current health state), but will ebb and flow. For example, if your TSH is 2.2 mIU/L at 4AM, it might be 1.4 mIU/L at 8PM – still within a healthy range, but different values.

While you cannot get your blood test done at 4am, getting it done at the first available morning appointment is considered the best time.

Getting your blood tests done after eating will also impact TSH levels. Eating, and even drinking coffee, are two things we do consistently that can decrease TSH secretion.

Moral of the Story

Booking your lab appointment first thing in the morning prior to eating or drinking is the best time to test TSH. Do this every time you get your blood tests (especially if you’re being monitored every 6-8 weeks for thyroid management), and you can more reliably compare the values on your labs.

Of course, “chasing the numbers” isn’t the only important consideration for thyroid management. Your symptoms are extremely important – how you’re feeling. Talk to your naturopathic doctor if you have questions or concerns about your thyroid or testing.




Lucke, C., et al. “Studies on circadian variations of plasma TSH, thyroxine and triiodothyronine in man.” European Journal of Endocrinology 86.1 (1977): 81-88.

Mahadevan, Shriraam, et al. “Does time of sampling or food intake alter thyroid function test?.” Indian journal of endocrinology and metabolism 21.3 (2017): 369.

Nair, Rakesh, et al. “Does fasting or postprandial state affect thyroid function testing?.” Indian journal of endocrinology and metabolism 18.5 (2014): 705.

Oliveira, Karen J., et al. “Thyroid function disruptors: from nature to chemicals.” Journal of molecular endocrinology 1.aop (2018).

Russell, Wanda, et al. “Free triiodothyronine has a distinct circadian rhythm that is delayed but parallels thyrotropin levels.” The Journal of Clinical Endocrinology & Metabolism 93.6 (2008): 2300-2306.

Sheehan, Michael T. “Biochemical testing of the thyroid: TSH is the best and, oftentimes, only test needed–a review for primary care.” Clinical medicine & research 14.2 (2016): 83-92.


Jenny Schmidt-White, ND

Hi, I’m Dr. Jenny Schmidt-White. My practice focuses on family health and wellness, healthy aging for men and women and optimizing fertility & hormone function. I work with you, within realistic and sustainable parameters, to find the root cause of dysfunction in your body and address it.

Leave a Reply