Seasonal thyroid fluctuations, biology and mood

 As you may have read from previous blogs, the thyroid, its pituitary stimulator - thyroid stimulating hormone (TSH) and the other thyroid hormones are heavily influenced by environment, nutrition and stress. Additionally these hormones can present as normal when relied upon purely by biochemical analysis from the blood. The seasons, differing temperatures, light exposure and effects of hibernation hormones and neurotransmitters can also be a key factor in the expression of adequate energy, organisation and coherence of an individual’s biology. We get sick more so in winter when our function is suppressed and the immune system is called upon to mount a response.


S.W. Tromp Biometerology 1967.

“ The yearly influenza peak in the Netherlands, around February. Which may be related to the usually low humidity and wind-speed in this period, but which effect is probably accelerated by the decreased thermoregulation efficiency of the body as a result of the preceding cold months and the accompanying changes in the physico-chemical state of the blood such as y-globulin level.”

 Ambient temperature can have a significant effect on TSH production in as much as a colder environment increases TSH and warmer temperatures decrease TSH production and thyroid requirement. Observations have suggested a biphasic seasonal nature of TSH secretion, with increased TSH readings during winter time suggesting what could be a functionally hypothyroid or subclinical hypothyroidism which resolved during the summer months (Kim et al., 2013). As this stress increases throughout longer days of darkness, organisational hormones decrease, whilst stress hormones increase. If chronic enough, or in an unstable biology, stress can decrease the accuracy of TSH to predict a low thyroid state

 Light, both red and ultraviolet (UV) are well-known modulators of immune function, metabolism and mitochondrial production of energy or  adenosine triphosphate (ATP) (Wong-Riley et al., 2005) (Karu, 2010). These aspects of sunlight, exert their influence via enhancement of aerobic metabolism (at cytochrome c) and immunity enhancing via infra-red (NIR) and vitamin D synthesized by UV respectively.  The variation in light exposure as a consequence of daily sleep, darkness and seasonal variations present relationships that may explain the secretory patterns of TSH in healthy subjects. More than 100 years ago, thyroid function could be suggestively viewed via uptake of thyroid iodine levels in seasonal variations. (Fenger and Siedell 1913). Thyroid iodine levels rose during the summer in sheep, pigs and cows and decreased during the winter reflecting the variations in the need for TSH/TH production in healthy organisms. 

Seasons, Thyroid and Mood

 Depression is a known symptom of hypothyroidism and some studies have highlighted the need for a lower TSH value in the presence of depressive symptoms (Talaei, Rafee, Rafei, & Chehrei, 2017) (Hage & Azar, 2012). The former authors suggest a cut-off value of 2.5 mU/L for TSH as a point for treating hypothyroidism, which highlights the need for assessing symptoms as part of an effective strategy for diagnosing hypothyroidism. This compares to the attitude taken to expecting mothers where values should be decreased to compensate for hypothyroid states but in reality should be applied across the board.

 Relationships concerning seasonal variations of mood are well documented and decreased Beck mood scores are associated with the shorter days of winter (Harmatz et al., 2000). Seasonal affective disorder (SAD) may be a reflection of the increase in serotonin and melatonin and depression of thyroid hormone, which are increased by shorter days and in mammals are associated with hibernation. This aspect seems to be lost on those treating transient depressed states but light therapy does appear to be taken seriously these days. I would encourage anyone wanting more information on serotonin and mood to check out the extremely well written blog Against Utopia.

 As days become shorter and light exposure is decreased, influencing cellular function and metabolism negatively. The extended effects of melatonin from the shorter days can antagonise TSH secretion via its inhibitory action on TH, increasing prevalence throughout winter. Whilst fluctuations in TSH levels in response to seasonal changes are well-known to occur, these fluctuations are also under the influence of the nutritional and environmental factors that can suppress TSH values.

Untitled design-3.png

Wake me up when it’s spring?

Violent suicides increase with the onset of spring from March to May. I was trying to think why this might occur? After speaking to a friend recently about depression, they said that holding onto the feelings of a blanketed, safe, dark environment by being it home (in a somewhat hibernation like state), and gorging on boxsets or podcasts was easy to do and a comfort. I wonder if the shorter phases of darkness and increased light remove that blanket of increased serotonin and melatonin and the light itself might become a stressor that takes away that comfort? Those most at risk might explain this seasonal increase in suicides?

Ray Peat (1997) has discussed various aspects of stressors such as darkness, oestrogen-cold sensitive enzymes and nutritional factors affecting endocrine systems, adding an interesting perspective on hormone production and relationships with temperature changes (Peat, R. and Soderwall, 1972) (Peat, 1997)(Peat, 1972).

In states of undetectable SCH mediated by the stress, a hypothermic state may stimulate the adrenal stress system to compensate for a low-metabolic and decreased temperature state.  Activated compensatory stress response pathways may explain poorly detected hypothyroid patients. Decreased metabolic rate, lowered temperature and pulse rate are well-known signs of hypothyroidism. β adrenergic mechanisms involving increased catecholamine production such as adrenaline and noradrenaline (NA) can increase Tb and RHR. In my previous blog on body temperature, I explained how low temperature can be indicative of low thyroid function when blood tests appear normal.



Hage, M. P., & Azar, S. T. (2012). The link between thyroid function and depression. Journal of Thyroid Research.

Harmatz, M. G., Well, A. D., Overtree, C. E., Kawamura, K. Y., Rosal, M., & Ockene, I. S. (2000). Seasonal variation of depression and other moods: A longitudinal approach. Journal of Biological Rhythms.

Karu, T. I. (2010). Multiple roles of cytochrome c oxidase in mammalian cells under action of red and IR-A radiation. IUBMB Life.

Kim, T. H., Kim, K. W., Ahn, H. Y., Choi, H. S., Won, H., Choi, Y., … Park, Y. J. (2013). Effect of seasonal changes on the transition between subclinical hypothyroid and euthyroid status. Journal of Clinical Endocrinology and Metabolism.

Peat, R. and Soderwall, A. L. (1972). Estrogen stimulated pathway changes and cold -nactivated enzymes. Physiol Chem Phys, 4((3)), 295–300.

Peat, R. (1997). From PMS to Menopause: Female Hormones in context.

Peat, R. (1999). Thyroid Therapies, Confusion and Fraud. Retrieved from

S.W., Tromp. (1967). Biometeorology, iia and b. Symp. Publ. Div. Pergamon Press (Oxford).

Talaei, A., Rafee, N., Rafei, F., & Chehrei, A. (2017). TSH cut off point based on depression in hypothyroid patients. BMC Psychiatry, 17(1).

The Armour Laboratories. (1945). The Thyroid Gland and Clinical Application of Medicinal Thyroid. Armour Laboratories.

Wong-Riley, M. T. T., Liang, H. L., Eells, J. T., Chance, B., Henry, M. M., Buchmann, E., … Whelan, H. T. (2005). Photobiomodulation directly benefits primary neurons functionally inactivated by toxins: Role of cytochrome c oxidase. Journal of Biological Chemistry.