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Response to thyroxine (levothyroxine sodium) is best monitored biochemically.
Thyroid function should be assessed every 6-8 weeks until the patient is
euthyroid and then rechecked annunally, aiming to maintain T4 and TSH within the
normal range (1). Elevated T4 with TSH suppression may suggest overtreatment - suppression
of TSH with serum T4 at the upper end of the normal range or even slightly elevated
is sometimes observed in those taking standard doses. These biochemical findings
are indicative of over-treatment and indicate a need for dose reduction; there
is evidence for long-term cardiovascular risk associated with mild over-treatment
(2)
In the community, undertreatment is more likely and annual measurements
of serum TSH may be valuable in assessing the adequacy of therapy and compliance - evidence
that up to 25% of patients in the community receiving T4 for hypothyroidism are
under-treated, so the adequacy of therapy and compliance should be checked by
annual serum TSH measurement. In non-compliant patients who take T4 for a few
days before the clinic, thyroid function tests typically reveal normal or even
elevated T4 with paradoxically raised TSH (2)
If the original diagnosis
of hypothyroidism is in doubt once thyroxine (levothyroxine sodium) therapy has
been initiated, then consider stopping treatment for 6 weeks and measure serum
TSH and T4 in the non-treated patient. Notes: - in the majority
of patients, dose requirements for T4 do not change
- however pregnancy often
necessitates a dose increase to maintain serum TSH within the normal range. Therapy
with some drugs also alters T4 dose requirements, because of effects on T4 absorption
or metabolism
- rifampicin, phenytoin, carbamazepine (increased clearance
of thyroxine)
- cholestyramine, sucralfate, aluminium hydroxide, ferrous
sulphate (reduced absorption of thyroxine)
Reference: - Prescriber
(2002); 13(10): 50-68.
- Franklyn J. Hypothyroidism. Medicine 2005; 33
(11): 27-29.
- Franklyn J. Concensus statement for good practice and audit
in the management of hypothyroidism and hyperthyroidism. BMJ 1996; 313: 519.
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