Thyroid hormone secretion is more sensitive than thyroid cyclic AMP accumulation to stimulation with LATS in mice in vitro and in vivo.

The effects of LATS-immunoglobulin G (IgG) on thyroid hormone secretion and on thyroid cAMP concentrations were investigated in mice and compared to those of TSH. In the in vitro experiments, thyroid lobes were incubated in Krebs-Ringer bicarbonate buffer with LATS-IgG or TSH for 3 h or 2 h and T3 concentrations in buffer and thyroid cAMP were measured by RIA. T3 in the buffer was increased with 1.5 mg/ml of LATS-IgG (A) or 2.5 mg/ml of LATS-IgG (B) (1000%/5 mg or 400%/5 mg in the McKenzie bioassay, respectively), whereas thyroid cAMP was elevated only after incubation with two to four times higher doses of LATS-IgG (A) or LATS-IgG (B). 0.03 mU/ml of TSH increased T3 concentrations, while a two fold higher dose of TSH was required to increase thyroid cAMP. In the in vivo study, 5 mg of LATS-IgG (A) injected intravenously increased serum T4 concentrations but not thyroid cAMP. 2 mU of TSH increased serum T4, while 10 mU was needed to elevate thyroid cAMP. These results indicate that: i) thyroid hormone secretion is more sensitive than increases of thyroid cAMP to stimulation with LATS, which is similar to stimulation with TSH and that: ii) thyroid hormone secretion rather than increases of thyroid cAMP should be employed to detect serum thyroid stimulating activities when mouse thyroids are used.