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Binary Switching of Calendar Cells in the Pituitary Defines the Phase of the Circannual Cycle in Mammals

Yet again, an interesting piece of research which could be of significance to most of us here, but that could easily be missed.

Not fully read yet, but will make my way through - decided to post as it so obviously would affect thyroid issues.

Binary Switching of Calendar Cells in the Pituitary Defines the Phase of the Circannual Cycle in Mammals

Shona H. Wood

, Helen C. Christian

, Katarzyna Miedzinska

, Ben R.C. Saer

, Mark Johnson

, Bob Paton

, Le Yu

, Judith McNeilly

, Julian R.E. Davis

, Alan S. McNeilly

, David W. Burt

, Andrew S.I. Loudon


•A circannual timer may reside in the pituitary pars tuberalis thyrotroph

•This is defined by a digital switching mechanism controlling EYA3 expression

•The circannual clockwork drives a morphogenic cycle in the PT and hypothalamus

•This involves recapitulation of a developmental program


Persistent free-running circannual (approximately year-long) rhythms have evolved in animals to regulate hormone cycles, drive metabolic rhythms (including hibernation), and time annual reproduction. Recent studies have defined the photoperiodic input to this rhythm, wherein melatonin acts on thyrotroph cells of the pituitary pars tuberalis (PT), leading to seasonal changes in the control of thyroid hormone metabolism in the hypothalamus. However, seasonal rhythms persist in constant conditions in many species in the absence of a changing photoperiod signal, leading to the generation of circannual cycles. It is not known which cells, tissues, and pathways generate these remarkable long-term rhythmic processes. We show that individual PT thyrotrophs can be in one of two binary states reflecting either a long (EYA3+) or short (CHGA+) photoperiod, with the relative proportion in each state defining the phase of the circannual cycle. We also show that a morphogenic cycle driven by the PT leads to extensive re-modeling of the PT and hypothalamus over the circannual cycle. We propose that the PT may employ a recapitulated developmental pathway to drive changes in morphology of tissues and cells. Our data are consistent with the hypothesis that the circannual timer may reside within the PT thyrotroph and is encoded by a binary switch timing mechanism, which may regulate the generation of circannual neuroendocrine rhythms, leading to dynamic re-modeling of the hypothalamic interface. In summary, the PT-ventral hypothalamus now appears to be a prime structure involved in long-term rhythm generation.

Full paper avaialble here:


The BBC's take on this doesn't mention the "thyroid" word even once. (And by mentioning "hormones" we are probably expected to assume oestrogen, progesterone and, maybe, testosterone - after all, they are what many think of as being hormones.)


3 Replies

Wow...clock genes ! ! !

I think we all recognise this as Season Affective Disorder but I don't really think it applies to oestrogen, progesterone and testosterone so much as thyroid hormones.

I have heard of members who actually take more thyroid hormone during the winter months.

I wonder what ACTUALLY drives this seasonal hormonal rhythm though... creating the various levels of melatonin...light ? warmth ? vit D ?

Haven't read full paper yet.



I agree about it not applying to oestrogen, progesterone and testosterone - but we so often see *any* mention of the word hormone being assumed to refer to them. Witness "hormone replacement therapy" - which is a decent enough description of taking thyroid hormone but is always used to refer to oestrogen and progesterone.

I have just increased my levothyroxine dose - but I don't know whether that was due to season or change of make!


...oh the nights are drawing in now Rod.....only THIRTEEN more Saturdays until christmas ... .(..we all wanted to hear that..didn't we .? ? ? ..)

.. time for a little raise..