The thyroid has been mentioned quite a bit in the media and science circles this year. Following the catastrophic effects of the earthquake and tsunami in Japan this year, images of the Fukushima Daiichi nuclear plant going into meltdown were projected across the world. The fear was that because radioactive particles of iodine (and quite a few other things) have been released and given that thyroid cells are the only cells in the body that absorb iodine, there might be some risk. A similar effect seemed to be realised after the Chernobyl meltdown where rates of thyroid cancer roundabout shot up.
The thyroid is a small gland in the neck whose key job is to convert iodine into thyroid hormones. These thyroid hormones in turn regulate the metabolism of things like proteins, fats and carbohydrates and energy and metabolic processes. The main thyroid pre-hormone is thyroxin (T4) which in the liver is activated to triiodothyronine (T3), the active hormone. Smallish amounts of other hormones are also formed in the thyroid gland. Thyrotrophin releasing hormone (TRH) is released from the hypothalamus when T3 and T4 levels drop, which in turn asks the anterior pituitary to release thyroid stimulating hormone (TSH) to release more T3 and T4. Another good example of our amazing bodily feedback capabilities.
When things go wrong with the thyroid it normally goes one of two ways: hypothyroidism, where hormone production is lower than it should be (primary or secondary), or hyperthyroidism, where lots of hormone is produced. Graves' diseases is an autoimmune condition and a common form of hyperthyroidism.
OK thyroid 101 is over. What about any connection to autism spectrum conditions?
A fairly recent paper by Hoshiko and colleagues* adds to what is a small but growing body of work examining the thyroid and its minion hormones in relation to autism. Based on the combined strength of two study groups based in California, USA, 544 cases of autism were compared with 784 matched controls, and an elevated risk of autism was suggested where T4 levels were very low. The caveat to this statement is that significance was only reached for one cohort born in 1995. Remember T4 is the prehormone.
I have touched upon thyroid function and autism before on this blog. In that post, there was a suggestion of a link between autism and flame retardant materials containing PBDEs. Looking at the other research in this area, the results could best be described as mixed. This paper for example, again examining neonatal thyroxine levels in various neurodevelopmental conditions failed to find any significant association. Indeed quite a few of the earlier studies in this area reported no overall association between thyroid hormone concentrations and autism despite some individual cases potentially being linked.
That being said, don't rule out any thyroid connection just yet. Jim Adams and colleagues reported a 45% reduction in iodine levels in their cohort of children with autism which they speculated might tie into some problems with thyroid function (see here for an overview of iodine deficiency). This finding was part of a wider issue with other elements and perhaps mirrors similar problems recently reported on. Likewise this paper suggested that for those cases of autism where regression was reported, there might be an association with a family history of autoimmune thyroid disease. I have a post scheduled soon on the regression/brain overgrowth findings in autism reported recently; perhaps thyroid issues should form part of further phenotypic investigations?
With these collected research in mind, together with other 'possible' connections such as thyroid function and reelin (mentioned in autism research), I wouldn't necessarily close the book on any links between autism and thyroid function just yet. Indeed the link between the thyroid and psychiatry is just starting to get quite interesting..
* Hoshiko S. et al. Are thyroid hormone concentrations at birth associated with subsequent autism diagnosis? Autism Research. August 2011.