Abstract
Approximately, 5% of the population is affected by hypothyroidism, mainly women and persons aged more than 60 years. After the diagnosis of hypothyroidism the usual therapy is tablet levothyroxine (L-T4), with a monitoring of the thyroid-stimulating hormone (TSH) level in primary hypothyroidism every 6–8 weeks and L-T4 is adjusted as necessary to reach an euthyroid state. Once TSH is stabilized in the normal range, it is recommended to conduct annual testing in the treated subjects to warrant suitable replacement. More recently advances regarding L-T4 treatment are the introduction of new oral formulations: the liquid solution, and soft gel capsule. The soft gel capsule permits a quick dissolution in the acid gastric pH. The liquid preparation does not require an acid gastric environment. Many pharmacokinetic studies demonstrated a more rapid absorption for the liquid L-T4, or capsule, than with tablet. Many studies have shown that the liquid, or capsule, formulations can overcome the interaction with foods, drugs or malabsorptive conditions, that are able to impair the tablet L-T4 absorption. Lately studies have suggested that liquid L-T4 can permit to maintain more efficiently normal TSH levels in hypothyroid patients in the long-term follow-up, than tablet L-T4, both in patients with malabsorptive states, and in those without malabsorption. Further large, prospective, longitudinal studies are needed to evaluate the stability of TSH, in hypothyroid patients treated with different L-T4 formulations.
Highlights
In physiological conditions, thyroid hormones (TH) are produced by the gland and their synthesis depends on normal iodide transport
A negative feedback controls the release of TH: Thyroid-stimulating hormone (TSH) secretion is suppressed if free T3 (FT3), or free T4 (FT4) levels are elevated, and the same happens for the production of thyrotropin-releasing hormone (TRH) by the hypothalamus [2]
The frequency of hypothyroidism is higher in women, especially over 60 years, and it can be diagnosed measuring serum TSH and free T4 values [2]
Summary
Thyroid hormones (TH) are produced by the gland and their synthesis depends on normal iodide transport. Resistance to TH (RTH) is a rare autosomal dominant disorder that leads to elevated free TH levels, in the presence of normal or increased serum TSH concentrations, if it is generalized because both the pituitary and peripheral tissues are partially resistant [20]. Once those are excluded, an altered intestinal absorption of L-T4 (caused by gastrointestinal disorders, some nutrients, or drugs) is considered the principal cause of refractory hypothyroidism. Albeit L-T4 dose and free T4 levels were comparable, patients taking liquid L-T4 had significantly lower TSH values in comparison to those receiving tablets, at 7–10 days (P=0.05) and 6–8 months (P=0.043) of therapy. It has been shown that the L-T4 solution can control better serum TSH values compared to tablets in elderly [30], and in pregnant women [31]
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