For compounds that have a secondary thyroid effect, as a consequence of increased
thyroxine clearance, changes in hormone levels are more modest. Seven days after the
administration of the inotropic drug, OPC 8212, serum T4 was reduced by about 20% but
TSH values were more than doubled (Lueprasitsakul et al., 1991). Similarly, McClain
(1989) was able to demonstrate that phenobar-bitone, by its liver microsomal enzymeinducing effect, also modified thyroid-pituitary homeostasis. After 2 weeks of dosing, a 30% fall in serum T4 was seen in male rats and this was accompanied by a doubling of serum TSH concentration. Over the following 10 weeks, serum TSH and T4
concentrations returned to near pre-study values although thyroid weights remained
elevated.