Relationship of water chemistry to serum thyroid hormones in captive sharks with goitres

Captive whitetip reef sharks, Triaenodon obesus, at Sea Life Park (SLP) Haw aii, Waimanalo, Hawaii develop goitre, whereas, T. obesus at the Waikiki Aq uarium (WAQ), Honolulu, Hawaii do not develop goitre. To determine the effe cts of natural concentrations of iodine on the reduction of goitre, two sha rks from SLP with goitre were placed in an enclosed coastal lagoon with nat ural seawater. Using ultrasound measurements the two goitres were initially 11.4 cm and 14.6 cm in depth and after 3 months decreased to 4.7 cm and 5. 7 cm. Radioimmunoassay analysis of the thyroid hormone T 3 sera concentrati ons were initially 0.22 ng/ml and 0.33 ng/ml and increased to 1.84 ng/ml af ter being placed in the lagoon. Sera T-4 were initially 0.93 ng/ml and 0.99 ng/ml and increased dramatically to 17 ng/ml and 56 ng/ml. Over the six mo nth sampling period, two resident sharks in the lagoon with normal thyroids had sera T-3 concentrations from 0.89 ng/ml to 1.1 ng/ml, and sera T-4 con centrations from 3.1 ng/ml to 7.9 ng/ml. The hypothyroid condition in the S LP goitred-sharks is likely linked to the low environmental iodide (<0.005 mu M), and high nitrate (111 mu M) of SLP water. The WAQ well-water facilit y was characterized by anoxic water with high iodide (0.60 mu M), total iod ine (1.90 mu M), and typical nitrate (24.6 mu M) concentrations of intersti tial groundwater. The difference of iodide concentration of SLP and WAQ tan k water (<0.005 mu M versus 0.60 mu M) was directly related to the hydrogeo logy of the well-water sources. To avoid goitre in marine aquarium systems, we recommend maintaining iodide concentrations of at least 0.15 mu M.