d-Triiodothyronine (d-T3) is used in the treatment of hyperlipidaemia. To examine the influence of this kind of therapy on thyroid function the effect of various doses of d-T3 on TRH induced TSH release has been studied in 10 male euthyroid subjects. d-T3 was administered orally for 4 days; blood samples were taken on the first day 12 h after the d-T3 dose and then after an additional 12 h and after 4 days. The final sample was taken on the 5th day after 24 h without d-T3. Administration of 31.25 μg of d-T3 reduced the maximum increase of TSH in response to 200 μg TRH iv by 40%; 62.5 and 125 μg of d-T3 did not produce a significantly higher inhibition (50%; 45%). However, after 250 μg of d-T3, the response to a single dose was reduced by 68 %. After 500 μg of d-T3 TRH did not stimulate TSH at all. After 4 days regardless of d-T3 dosage the inhibition was not significantly lower than after 24 h. Prolonging the administration of 500 μg d-T3 over 3 weeks raised PBI to 150% whereas thyroxine fell to 65% of the values before d-T3 administration. In a primary hypothyroid patient the basal TSH secretion was lowered from 63 to 36 μU/ml by a daily dose of 0.5 mg d-T3 for 12 days. This study demonstrates that TSH release is inhibited by small doses of d-T3. This stresses the importance of assessing thyroid function in patients receiving d-T3.