IN the twenty-ninth Bedson Lecture delivered on February 8 in Newcastle, Prof. C. R. Harington dealt with the relation of the thyroid gland to iodine. He traced in detail the parallel histories of the anatomy, physiology and pathology of the gland, culminating in the work of Kocher, the treatment with sheep gland extracts by Murray, and the iodine treatment by Coindet of goitre and cretinism. Although the 3: 5-diiodotyrosine also present in the ‘colloid’ must take part in the activity, as this is proportional not to total thyroxine but to total iodine, the two di-peptides made from them have not proved to show the full activity, so that possibly they are linked to, or by, other amino-acids, it being fairly established that no other compound of iodine is present. The general picture, then, is as follows. Iodine is readily taken up by the gland with formation of 3: 5-diiodotyrosine, which is elaborated into the globuline; this is the storage form, the so-called ‘colloid’, of the epithelial layer. When total iodine in the gland falls below 0-1 per cent, the colloid is soon exhausted and the epithelium extends to form goitre. Later, generally in pre-natal conditions, atrophy occurs leading to cretinism. Restoration of iodine at the former stage leads to distension by colloid, and the epithelium reabsorbs. In the normal state the tyrosine derivative is partly converted into thyroxine, and these two substances form the hormone which regulates bodily metabolism in general. In support of this view, it has been shown by Prof. Harington that the tyrosine and thyroxine are of the same stereochemical configuration by the preparation from each of thyronine (desiodo-thyroxin).