Objective: To determine the 24hour thyroid hormone profile on patients with combined T3/T4 therapy.
Methods: Two subgroup of patients (10 on combined T3/T4 (study, S) and 10 T4 alone (control, C) were randomly selected from 697 patients who were already involved in WATTS study, after ethical approval. Prior to study entry all patients were on 100micrograms or more of T4. The study group received 10micrograms of T3 in place of 50micrograms of T4 while the T4 dose in the control group remained unchanged. Blood samples were collected for T3, T4 and TSH at 0, 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 22, and 24 hours post dose.
Summary: Basal free T3 levels were similar between the 2 groups (S:C 4.384:4.69 picamoles per litre, p equals 0.208), but basal mean T4 was lower(S:C 12.05:17.9 picamoles per litre, p less than 0.001) and basal mean TSH was higher(S:C 5.3:1.0 milliunits per litre, p equals 0.01) in the study group. Profiles of all 3 hormones were very stable over the 24hour period on the control group but FreeT3 levels showed a 42% rise within the first 4 hours of medication ingestion and remained higher than the control group for 16 hours. The mean Free T3 levels at 4 hours post dose (S:C 6.24:4.63 picamoles per litre, p equals 0.007)as well as the area under the curve(S:C 1148:1062, p less than 0.0001) were significantly higher in the study group.
Conclusions: Despite chronic combined T3/T4 therapy, wide trough to peak variation in Free T3 levels present in patients on combined T3/T4 therapy but not on T4 alone.