Acute effect of exogenous thyroxine dose on serum thyroxine and thyrotrophin levels in treated hypothyroid patients.

Acute effects of exogenous thyroxine on the serum thyroxine (T4), free thyroxine (FT4) and thyrotrophin (TSH) levels were studied in 20 hypothyroid women during the long-term maintenance therapy. All patients were euthyroid by clinical and laboratory parameters (T4). In nine patients TSH secretion was totally suppressed (less than 1 mu/l) by the mean dose of 178 micrograms thyroxine. Eleven patients had TSH values greater than 1 mu/l. Their mean thyroxine dose (132 micrograms) was not significantly lower than that which totally suppressed the TSH secretion. Ingestion of thyroxine resulted in maximal T4 and FT4 levels at 2 h after taking the drug. Simultaneously TSH concentration reached its minimum. The results indicate that it is necessary to know when the patient has taken the thyroxine in relation to determination of T4, FT4 and TSH levels in order to make the right decisions about the substitution dose of thyroxine. Thus it is essential to take blood sample for hormone determination before the patient ingests the daily thyroxine dose. However, among hypothyroid patients substituted by thyroxine there are individuals whose TSH secretion is not suppressed by normal T4 and FT4 levels.