The primary goals of treatment of primary hypothyroidism are improvement of signs and symptoms (improvement time may be different for each clinical finding), normalisation of free T4, T3, and TSH, which may usually take longer.
Levothyroxine has a half-life of about 7 days. A steady state between T4 and TSH concentrations is generally achieved about 6 weeks after treatment begins.
In cases of severe hypothyroidism, or in patients with special conditions requiring closer monitoring of thyroid function, the assay of TSH and free T4 should be performed 4 to 6 weeks after the start of treatment. This interval is extended as we approach a normal situation. In the long term, once the biochemical parameters have been normalised, an annual or semi-annual assay of TSH will be sufficient to monitor therapy.
Clinical findings must be taken into account when adjusting therapy, but doses should not be adjusted without biochemical assays (ideally TSH and free T4).
Note that in pregnant women normal TSH values vary per trimester, considering the following upper limits of TSH normality:
- 1st trimester - 0,1 - 2,5 µU/L
- 2nd trimester - 0,2 - 3,0 µU/L
- 3rd trimester - 0,3 - 3,0 µU/L
Specific cut-offs of total T4 or free T4 should be used in pregnant women. Pregnant women require higher than usual doses, with recommended increments of 30% or more. Treatment is more closely monitored in children and pregnant women and is done monthly. In the first year of life, biochemical monitoring is required every 4 to 8 weeks. In the first half of the pregnancy, we suggest biochemical monitoring every 4 to 8 weeks. We recommend that children and pregnant women should be treated in centres that specialise in paediatric endocrinology.
Normal TSH values are slightly higher in the elderly.
