In Winter, everyone gains T3 except thyroidless patients on LT4 therapy

In this article I examine the research results depicted in the graph above.

Gullo and team’s 2017 article reported research on 11,806 healthy controls and 3,934 thyroidless people treated with L-T4 monotherapy in Sicily, Italy.

They discovered that the thyroid-disabled people lost a “significant” amount of Free T4 and Free T3 hormone in winter, on average, in addition to a loss of T4 that caused an increase in TSH.

In contrast, people with healthy thyroids experience the very opposite effect — a statistically significant increase in average Free T3 during the colder months of the year. Unlike the thyroidless, healthy controls kept their average TSH and FT4 levels almost exactly the same all year long.

This seasonal effect in winter exaggerates the average Free T3 deficit in people treated with T4 hormone alone.

Even in summer, the thyroidless on LT4 already have significantly lower Free T3 than healthy controls. Thyroid scientists all agree, and we have known for many decades, that their year-long significantly lower T3:T4 ratio is induced by T4 monotherapy itself, not by illness.

But when winter comes, the difference is even more extreme between the lower FT3 levels of thyroidless T4-monotreated people and the higher FT3 of healthy people.

Specifically, in the longitudinal study, the healthy controls increased FT3 from 4.31 pmol/L in the warmest four months of Summer to 4.40 pmol/L in the coldest four months of Winter, while the thyroid-disabled decreased FT3 from 4.07 in Summer to 3.80 pmol/L in Winter.

Because the thyroidless patients already had a lower FT3 in summer, when the Winter came, thyroid-disabled patients had a net deficit of FT3 compared to healthy controls. The thyroidless patients’ relative loss of FT4 in winter still kept their FT4 higher than healthy controls at a surplus.

In this post, I’ll provide an overview of the data they presented. 

I’ll also calculate their data as “percent of reference range” so that they can be more easily quantified and translated (approximately) to other FT3 and FT4 assays with different reference ranges.

I’ll discuss some of the implications of a lower FT3 in winter.

Despite its limitations, this research matters.

The study did not have a methodology that could examine the “clinical significance” of the data, that is, whether the FT3 loss worsened any of the patients’ symptoms or changed their body temperature or affected any concurrent medical conditions.

It also had some shortcomings in terms of not revealing the full spread of the data, including statistical outliers, and not revealing any examples of individuals’ FT3:FT4 ratios over the seasons.

Nevertheless, the study questions common theoretical assumptions about the stability of the HPT axis during thyroid therapy and the degree to which it is distorted and modified by not having a thyroid and being on levothyroxine (LT4) monotherapy.

This research data and its discussion is important because the HPT axis model of health, and fixed reference ranges, are currently being used to judge the “euthyroid” status of patients on this therapy at all seasons of the year, as if there are no fluctuations.

The study has significant clinical implications because patients at the boundaries of the current diagnostic criteria can be misdiagnosed, underdosed, or overdosed based on the theory’s assumptions about seasonal stability. Therapy is also “monitored” by the idea that an individual may only need testing for thyroid dose adjustments once a year.

If the data on such a large population show that this supposedly unchanging axis undergoes significant distortions, it has potential implications for scientific studies of thyroid therapy performed at different times of year.

In general, the theory of the human HPT axis during thyroid loss and thyroid therapy needs to be refined and updated to account for seasonal shifts and distortions.

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Their data

W = Winter, their coldest 4-month period, December to March
S = Summer, their warmest 4-month period, June to September.

Cross-sectional study:

Euthyroid subjects (or “HC,” healthy controls)

  • W: 3,819 people.  S: 3,703  people.

Hormone (units) W/S average (IQR, interquartile range, central 50% of data)

  • TSH (mU/L) W = 1.41 (0.90-2.18); S = 1.40 (0.90-2.14)
  • FT4 (pmol/L) W = 14.0 (12.4-15.4); S = 13.9 (12.2-15.4)
  • FT3 (pmol/L) W= 4.47 (3.96-5.00); S = 4.34* (3.85-4.93)
  • FT3/FT4 ratio W = 0.32 (0.28-0.36); S = 0.31 (0.27-0.36)

LT4-treated athyreotic patients (“Rx”)

  • W: 1,315 people. S: 1,241 people


  • TSH*** (mU/L) W = 0.70 (0.20-1.68); S = 0.38* (0.10-1.20)*
  • FT4 (pmol/L) W = 16.4 (14.2-18.4); S = 16.7* (14.3-19.3)
  • FT3 (pmol/L) W = 3.86 (3.39-4.44); S = 4.00** (3.47-4.47)
  • FT3/FT4 ratio W = 0.23 (0.20-0.25); S = 0.24 (0.20-0.27)
  • LT4 dose (μg/kg/d) W = 1.64 (1.39-1.86) S = 1.65 (1.42-1.91)

Statistical significance of change from W to S: *P = <.001, **P <.008 (from text, p. 211)

“In the athyreotic patients, the slope of the FT4/TSH inverse correlation was much steeper in summer (−1.52) than in winter (−0.85) (P<.001, ANCOVA test).”

NOTE: TSH was suppressed by therapeutic policy in many of the Rx patients because all had been diagnosed with thyroid cancer. Their total thyroidectomy, with or without subsequent radioiodine treatment, had occurred at least 1 year before their data was collected, and their dose of LT4 was stable and constant throughout the study.

Longitudinal study

As reported in this same article, researchers also conducted a more rigorous and in-depth “retrospective longitudinal study” of data.

They focused on 119 of the athyreotic patients for whom they had two full thyroid hormone tests of TSH, FT3 and FT4 during both the coldest and hottest months of the same year.

Their data was matched for comparison with 159 age-matched controls who also had similarly rich data per patient.

In the longitudinal study, they noticed that “the seasonal difference was more substantial” than the cross sectional study data.

Even the values the FT3 change from winter to summer (which had achieved a somewhat lesser significance calculation of P= <0.008 in the cross sectional study), achieved the statistical significance of P=<0.001 for the degree of change from winter to summer.

I used this data set to create the image at the head of this article. I provide it here once again so it can be viewed closer to the numeric data below.

Euthyroid subjects / HC

  • W: 159 people.  S: 159  people.

Hormone (units) W/S average (IQR, interquartile range, central 50% of data)

  • TSH (mU/L) W = 1.41 (0.90-2.40); S = 1.43 (1.00-2.33)
  • FT4 (pmol/L) W = 14.2 (12.5-15.4); S = 14.0 (12.1-14.9)
  • FT3 (pmol/L) W = 4.40 (3.88-4.84); S = 4.31 (3.85-4.77)
  • FT3/FT4 ratio W = 0.32 (0.27-0.36); S = 0.31 (0.27-0.36)

LT4-treated athyreotic patients / Rx

  • W: 119 people. S: 119 people.
  • TSH (mU/L) W = 0.80 (0.22-1.44); S = 0.20* (0.06-0.70)
  • FT4 (pmol/L) W = 16.3 (14.2-17.7); S = 17.8* (15.4-19.9)
  • FT3 (pmol/L) W = 3.80 (3.36-4.19); S = 4.07* (3.80-4.51)
  • FT3/FT4 ratio W = 0.23 (0.20-0.24); S = 0.24 (0.21-0.26)
  • LT4 dose (μg/kg/d) 1.59 (1.30-1.80) 1.59 (1.30-1.80)

Statistical significance of change from W to S: *P = <.001

“Percent of reference range” can translate research to clinical settings

Despite the many controversies that surround the methods and uses of reference ranges, they provide a common frame of reference to aid interpretation.

Unfortunately, reader cannot assume Gullo’s FT3 of 4.40 pmol/L is equal to one’s own 4.40 pmol/L in Calgary, Berlin, or Tokyo, because one assay may range from 2.8 to 5.9 and another may range from 3.5 to 6.5 pmol/L.

However, a patient from Calgary can say “my FT3 is at 50% of reference and my FT4 is at 80% of reference” and the hormones’ relationship may be immediately understood by someone who is used to a different FT3 reference range.

Similarly, an individual’s reduction in FT3 levels from 5.1 to 4.7 from summer to winter means something very different depending on the assay’s range.

But If one’s says “my FT3 fell by 20% of its reference range, and is now at 6% of range” the magnitude of the change and its potential significance is more immediately comprehensible.

All one needs to do is imagine the healthy population’s 95% reference interval for a substance as a bar from 0 to 100%.

Although “percent of reference range” may be unfamiliar to some people, is easier for the average patient and physician to comprehend than the more technical methods of standard deviations from the mean, or percentiles that exclude the upper and lower 2.5%.

Reference ranges for FT3 and FT4 vary widely between assay manufacturers and laboratories. This is not because they are poor quality tests. Indeed, they are sufficiently precise and reliable for clinical purposes and research purposes, given their low coefficients of variation reported in many research articles, and given the in-depth evaluation by an international committee more than ten years ago (Thienpont et al, 2010). The lack of calibration to international standard, while it causes confusion across regions, does not undermine each test’s technical quality.

Therefore, to aid clinical translation, I obtain reference ranges for the hormones and then provide their data as “percent of reference.”

Gullo’s reference ranges

Unfortunately, Gullo et al, 2017 did not provide local laboratory reference ranges for FT3 or FT4, but fortunately, they published an article in 2011 that included their ranges.

  • The FT3 range was 3.1 pmol/L wide (2.9 to 6.0 pmol/L).
  • The FT4 range was 10.6 pmol/L wide (9.0 to 20.6 pmol/L).

Their 2011 study’s ranges must also apply to the data in the 2017 study, since they said that “The reference ranges of the applied assays did not change during the period of our study,” and the term of their study was 2004 to 2014.

Differences between LT4 (Rx) patients and healthy controls

The “longitudinal study” data is the basis of these comparisons because its data involved controls matched for age as well as at least one winter and one summer value for each patient.

Free T3

  • Healthy controls had a seasonal gain of 0.09 pmol/L in winter, which constituted a boost of +2.9 % of the reference range.
  • LT4-treated patients had a seasonal loss of 0.27 pmol/L in winter, which constituted -8.7% of the reference range.
  • T3 Inequity: The treated patients had 0.60 pmol/L less FT3 than healthy controls in winter, on average. This constituted -19.4% of the reference range.

Free T4

  • Healthy controls had a seasonal gain of 0.2 pmol/L FT4 in winter, which constituted a boost of +1.9 % of the reference range, which was insignificant, within the range of variation expected from the laboratory test technology.
  • LT4-treated patients had a seasonal loss of -1.5 pmol/L FT4 in winter, which constituted -13% of the reference range.
  • T3 Inequity: The treated patients had an average of 2.1 pmol/L more FT4 than healthy controls in winter, on average. This constituted +19.8% of the reference range.

FT3:FT4 Ratio

The top of ratio’s IQR Rx W interquartile range (0.25) was significantly lower than the bottom of the average ratio for HC W.

The ratios in the two populations were entirely different. They do not overlap even in summer.


The graphs in Gullo and team’s Figure 2 revealed that some of the Rx patients had a fully suppressed TSH (it was not reported how many), while all the healthy controls’ TSH data fell within reference.

This difference alone would have biased the LT4 treated patients’ TSH averages and IQRs to be lower than the healthy controls.

Therefore, the comparison of averages and “% of reference” between the two cohorts is not very helpful because their averages were not based on the same total range. They didn’t have the same common denominator.

On the other hand, the visual incline of the “slope” of the FT4 correlation plotted across TSH values shown in their Figure 2 is a fair comparison between the two cohorts and is both statistically and biologically significant in its implications for the HPT axis theory.

How does the human body respond to this lower FT3:FT4 ratio in blood?

The first factor to consider is that loss of circulating T4 and T3 are not metabolically equivalent.

T4 is not merely a “less active” form of T3, and not merely an inactive pro-hormone. Each is a different hormone with unique signaling capacity at different receptors:

  1. In health, T3 performs the vast majority of thyroid hormone signaling. T4 has 10-15% the affinity of T3 to the main thyroid hormone receptor in the nucleus, but such measurements were attained in laboratory experiments, not in vivo.
  2. T4 does not directly co-regulate our mitochondria, but T3 and 3,5-T2 do.
  3. Free T4 hormone within the reference range has activity at the integrin receptor on the cell membrane. However, Free T3 binds to this receptor only at levels above the reference range (Davis et al, 2019).
  4. At the integrin receptor, T3 can bind to a site on the receptor that sends a different signal than T4’s binding site.

Next, consider that the circulating portion of hormone is metabolically significant despite variation in “intracellular” T3 and T4 levels.

Both T3 and T4 are actively carried into and out of cells on transport proteins.

The T3 concentration varies from tissue to tissue and cell to cell, but all cells contribute to, and are sensitive to, circulating levels of T3.

  • In D2-expressing cells, when T4-T3 conversion is highly upregulated, T3 concentrations may be higher inside cells than in circulation.
  • Conversely, in D3-expressing cells, T3 concentrations may be lower inside cells than in circulation.
  • In D1 expressing cells, the activity of D1 is directly proportional to the signaling of T3 in its nucleus, and such cells exchange T3 easily with circulation.

Overall, the net FT3 in concentration is modified by all three deiodinase activities in all tissues, plus T3 secretion from the thyroid gland.

Therefore, a lower T3 level in blood represents lower T3 in tissues:

“As T3 is largely created intracellularly and contributes to the circulating T3 pool following its active transport across the plasma membrane, reduced T3 levels in the circulation are likely to reflect T3 deficiency within the bulk of the T3-producing tissues.”

(Hoermann et al, 2015)

Next, the portion of T3 that interacts directly with cells is its Free portion, not the Total portion.

The FT3 measurement is more metabolically significant than Total T3.

Symptoms and signs can be expected to correlate more strongly with Free T3.

The ratio of “bound vs. free” T3 and T4 hormones is affected by fluctuations in estrogen, and T3 is more significantly influenced by changes in albumin and when blood thinners are used.

Despite Total T3 and T4 fluctuations, the healthy human body ensures that the free portion is appropriate.

Outside of nonthyroidal illness when FT3 can be falsely inflated by blood thinners like heparin (Laji et al, 2001), it is not necessary to measure Total T3. We rarely measure Total T4 anymore due to its inaccuracies. For similar reasons, there’s no need to measure Total T3 when FT3 tests have been equal in precision and reliability for more than a decade, despite the difficulty of getting manufacturers to agree to calibrate their tests to the international standard (Thienpont et al, 2010).

Next, a higher-normal FT4 hormone concentration can present metabolic challenges to some thyroid-disabled bodies.

  1. On average, the global rate of T4-T3 conversion reduces as thyroid function is lost (Midgley et al, 2015). Fortunately, higher levels of FT3 can be achieved in most people by escalating the dose. Unfortunately, thyroid function loss is a significant handicap for some people. A minority cannot achieve levels of FT3 that remove hypothyroid symptoms while TSH is normalized (Larisch et al, 2018)
  2. Even among patients with total thyroidectomies, some thyroid patients are extremely “poor converters” of T4 hormone, while others are “good” or “intermediate” converters (Midgley et al, 2015).
  3. If the individual human body has any metabolic handicaps in D1 and D2 genes, removal or destruction of the thyroid gland, the human tissue with the richest expression of D1 and D2, can reveal this metabolic weakness.
  4. In addition to thyroid function loss, female sex and age reduce T4-T3 conversion rates further, rendering higher-normal setpoints less accessible to some individuals (Larisch et al, 2018). If a healthy thyroid gland were present, a higher TSH could have enabled them to achieve higher FT3 levels or at least maintain their individually-euthyroid FT3 level.
  5. T4 converts to T3 via deiodinase type 2 (D2) far less efficiently as FT4 rises in reference range and beyond. (See “Ubiquitination: The glass ceiling of T4 monotherapy.”) As FT4 rises, the body without a functional thyroid must rely increasingly on D1 in the liver and kidney for T4-T3 conversion. This can present a challenge for people with poor D1 genes or liver and kidney health problems.

A higher-normal FT4 presents health risks even in thyroid-healthy populations:

  1. The upper half of the FT4 reference range carries a higher cardiovascular risk. (See “Prevalence rates for 10 chronic disorders at various FT4, TSH and FT3 levels.”)
  2. Combining a high FT4 with a low FT3 can add mortality risk to many chronic diseases (See “Ataoglu: Low T3 in critical illness is deadly, and adding high T4 is worse.“)

Even if a patient could become symptom-free and avoid health risk at higher levels of FT4, they may not be able to achieve those higher levels within current TSH-centric treatment policies.

  1. Despite pituitary TSH sensitivity to FT4, pituitary TSH secretion is insensitive to an abnormally low FT3:FT4 ratio in levothyroxine therapy. This creates a significant HPT axis abnormality called the TSH-T3 disjoint. For every unit of TSH gained, a treated, thyroidless patient has less T3. Therefore, a TSH of 1.5 mU/L does not achieve the same thyroid hormone status in a person with and without thyroid function.
  2. Some patients are so poor converters of T4 that their FT3 is still below the population mean not only when TSH is normal but also when TSH is suppressed (Larisch et al, 2018). If a mid-range FT3 level is insufficient for the individual’s body, the patient can suffer disabling hypothyroid symptoms even at the point of pituitary TSH suppression. Therefore, the poor converter of T4 hormone is at risk of tissue T3 insufficiency if doctors prevent TSH from falling below reference.


The main thing one should notice in all this data is the degree of wintertime FT3 loss and the year-round FT3 inequity that exists between thyroidless on LT4 and the “healthy” population.

This data alone should explode any false impression that there is one single HPT axis that applies to all people across the hormone-shifting conditions of thyroid disease and therapy.

Physicians should be aware that seasonal change presents thyroid-disabled people with significant challenges to their euthyroid status if they have narrow individualized FT3 therapeutic windows. As Gullo and team say, patients may easily fall below their therapeutic window in winter or above it in summer:

“physicians and endocrinologists treating athyreotic patients should consider this evidence and evaluate the risk of undesired hyperthyroidism in summer and of mood deterioration related to thyroid hormone decrease in winter.”

Scientists should replicate this study in a climate located at a farther distance from the equator, since

“these changes may be greater and more frequent in areas where climatic seasonal differences are more marked.”

Future clinical research going forward ought to be based on a theory of the HPT axis that incorporates seasonal change in treated patients.

  • In studies of treated patients’ thyroid symptoms, were most surveys performed in winter when FT3 would be lowest and hypothyroid symptoms would be most severe?
  • In studies of T4-T3 combination therapy, was the experiment performed in summer when FT3 was highest and symptoms of overdose would be most severe?

My goal in this post has mainly been to share a treasure-trove of data that can ought to be analyzed and pondered by others, and which I can draw on later, too.

I’ll follow up later with further posts that may draw on this data set and the researchers’ own discussion of it.


Gullo, D., Latina, A., Frasca, F., Le Moli, R., Pellegriti, G., & Vigneri, R. (2011). Levothyroxine Monotherapy Cannot Guarantee Euthyroidism in All Athyreotic Patients. PLoS ONE, 6(8).

Gullo, D., Latina, A., Frasca, F., Squatrito, S., Belfiore, A., & Vigneri, R. (2017). Seasonal variations in TSH serum levels in athyreotic patients under L-thyroxine replacement monotherapy. Clinical Endocrinology, 87(2), 207–215.

Laji, K., Rhidha, B., John, R., Lazarus, J., & Davies, J. S. (2001). Abnormal serum free thyroid hormone levels due to heparin administration. QJM: Monthly Journal of the Association of Physicians, 94(9), 471–473.

Larisch, R., Midgley, J. E. M., Dietrich, J. W., & Hoermann, R. (2018). Symptomatic Relief is Related to Serum Free Triiodothyronine Concentrations during Follow-up in Levothyroxine-Treated Patients with Differentiated Thyroid Cancer. Experimental and Clinical Endocrinology & Diabetes: Official Journal, German Society of Endocrinology [and] German Diabetes Association, 126(9), 546–552.

Midgley, J. E. M., Larisch, R., Dietrich, J. W., & Hoermann, R. (2015). Variation in the biochemical response to l-thyroxine therapy and relationship with peripheral thyroid hormone conversion efficiency. Endocrine Connections, 4(4), 196–205.

Thienpont, L. M., Van Uytfanghe, K., Beastall, G., Faix, J. D., Ieiri, T., Miller, W. G., Nelson, J. C., Ronin, C., Ross, H. A., Thijssen, J. H., Toussaint, B., & IFCC Working Group on Standardization of Thyroid Function Tests. (2010). Report of the IFCC Working Group for Standardization of Thyroid Function Tests; part 2: Free thyroxine and free triiodothyronine. Clinical Chemistry, 56(6), 912–920.

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