Pharmacology of Thyroid Hormones
Thyroid hormones are used to treat primary hypothyroidism and hypothyroidism secondary to hypopituitarism. Here, we review two hormones – levothyroxine and liothyronine – in the treatment of these two disease states.
The thyroid gland is responsible for a variety of metabolic and cognitive processes. In states of hypothyroidism – also known as ‘underactive thyroid’ – patients may experience the following symptoms:
- Weight gain
- Poor memory / concentration
- Inability to tolerate cold temperatures
Diagnostic tests – such as measuring thyroid-stimulating hormone (TSH) and thyroxine – can establish whether the patient has the condition.
Here, we focus on the synthetic forms of thyroid hormones to treat hypothyroidism.
Mechanism of action
The thyroid gland produces thyroxine, T4, which is converted to the more active form T3, or triiodothyronine. Hypothyroidism treatment is about replenishing these thyroid hormone stores, usually over the long-term.
There are two main thyroid hormones to consider:
- Levothyroxine – synthetic T4
- Liothyronine – synthetic T3
Liothyronine has a quicker onset of action (a few hours) and offset of action (1-2 days) than levothyroxine. In other words, liothyronine has a shorter half-life. For this reason, liothyronine is often given in emergency cases of hypothyroidism.
Side effects associated with thyroid hormones include:
- Weight loss
- Difficulty tolerating heat
- Difficulty sleeping
Side effects are mostly related to excessive or high doses and resemble symptoms of hyperthyroidism.
When we talk about the clinical pharmacology of thyroid hormones, we need to think about the following factors:
- That thyroid hormones should be avoided in patients with cardiovascular disease as they can trigger serious events, such as heart attack.
- That if hypopituitarism is the underlying cause, patients are typically administered corticosteroids before thyroid hormone therapy due to the risk of Addisonian crisis.
- That gastrointestinal absorption of levothyroxine is reduced by antacids, calcium and iron salts. A 4-hour gap is typically advised.
- CYP inducers – such as carbamazepine and phenytoin – may necessitate a dose increase. Other drugs, such as tricyclic and tetracyclic antidepressants, can increase the risk of levothyroxine toxicity.
- Patients should be counselled to be aware of toxic symptoms – shakiness, diarrhea etc. (see above for more) – and consult a doctor if these symptoms manifest.
The purpose of hormone treatment is to restore a euthyroid state (normal thyroid hormone levels) in a way that works long-term for the patient. Patients may be on thyroid therapy for many years, often for life, so understanding treatment options is essential.
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