Johnston & Eagles (Reference Johnston and Eagles1999) report a prevalence of hypothyroidism, in terms of the indication for thyroxine treatment, under long-term lithium therapy which by far exceeds the estimated prevalence of clinical hypothyroidism in the local population (Aberdeen area). Like Kirov (Reference Kirov1998), who also retrospectively found a similar prevalence in a lithium-treated cohort from London, they compare their findings with results reported from North America and Sardinia. Whereas the Italian researchers (Reference Bocchetta, Cherchi and LoviselliBocchetta et al, 1996) did not find an excess of hypothyroidism under long-term lithium therapy, results of studies from the USA and, above all, from Canada are close to those from the UK, suggesting a considerable excess of clinical hypothyroidism under lithium treatment.

The well-known discrepancies in results among studies of lithium's anti-thyroid effects that have emerged frequently in different parts of the world over 30 years may not only be due to different study designs. In Canada, there is an overabundance of nutritional iodine (Reference Dussault, Delange, Dunn and GlinoerDussault, 1993); Italy is an iodine-deficient country. As in the general population (Reference Laurberg, Pedersen and HreidarssonLaurberg et al, 1998), in patients treated with lithium, ambient iodine seems to play a major role in the manifestation of thyroid failure. Conversely, iodine deficiency may act as a protective factor under lithium therapy. In iodine-deficient Germany, Italy and Spain, an excess of clinical hypothyroidism in patients taking lithium has never been reported, whereas in Canada, six papers from different clinics consistently reporting high prevalence of hypothyroidism under lithium therapy have been published (Reference LeutgebLeutgeb, 1999). Sorting the studies published on this topic geographically provides a confirmation of the early (case-report-based) assumption by Shopsin et al (Reference Shopsin, Shenkman and Blum1973) of a synergism between iodine and lithium in the manifestation of thyroid failure.

Consequently, in those countries where the World Health Organization's iodisation programme is gaining ground (Reference Dunn, Reiners and WeinheimerDunn, 1998), psychiatrists should be aware of an increased risk of clinical hypothyroidism in their patients taking lithium.