Thyroid arteries were studied in 37 Japanese fetuses from 4 months to 9 months old. The results were as follows:
(1) The occurrence of pyramidal lobe was 48.6%.
(2) The patterns of the branches arising from the aortic arch were classified into four types. Type A was the most common type and was found in 91.9%. Anomalous types C, G and CG were found in only one each (2.7%).
(3) In Type A, the arterial distribution to the thyroid gland was as follows; 1) The superior thyroid artery was found to be a single branch from the parent artery in 100% of cases on the right and in 94.1% on the left, in 2 cases it was found to be a thyro-lingual trunk on the left side. 2) The superior thyroid artery arose from the external carotid artery in 61.8% of cases on the right and in 47.0% on the left, from the divergent portion of the internal and external carotid arteries in 26.5% on both sides and from the common carotid artery in 11.7% on the right and in 26.5% on the left. 3) The inferior thyroid artery arose from the thyrocervical trunk in 93.9% on the right and in 84.8% on the left. The inferior thyroid artery was found to be absent in 2 cases (6.1%) on the right and in 5 cases (15.2%) on the left. 4) The thyroid ima artery was found in 15.2%. It arose from the brachiocephalic trunk in 4 cases and from the right common carotid artery in 1 case.
(4) In the 3 anomalous types C, G and CG, the arterial distribution to the thyroid gland was as follows; 1) Type C was found in a 8-month-old fetus. The superior thyroid artery originated from the external carotid artery. The right inferior thyroid artery was absent and the left inferior thyroid artery was derived from the thyrocervical trunk. The thyroid ima artery branched from the brachiocephalic trunk. 2) Type G was found in a 7-month-old fetus. The superior thyroid artery originated from the external carotid artery. The inferior thyroid artery arose from the thyrocervical trunk. The thyroid ima artery branched from the right common carotid artery. 3) Type CG was found in a 8-month-old fetus. The superior thyroid artery arose from the common carotid artery. The inferior thyroid artery originated from the subclavian artery to be a single branch. 4) The anomalous arterial distribution to the thyroid gland was recognized in 3 cases of the anomalous branching type of the aortic arch.
(5) The thyroid arteries was most frequently originated in the course of development from the ventral aortic root near the third aortic arch and the subclavian artery outside of the vertebral artery or the internal thoracic artery.
Based upon these results, it was considered that the thyroid arteries being distributed from the inferior portion were affected by the dynamic modification of the fourth aortic arch.
