Thyroid medicines explained: underactive and overactive thyroid

An underactive thyroid: replacing the missing hormone

In an underactive thyroid (hypothyroidism), the gland does not make enough thyroid hormone, leaving you tired, cold, low in mood and prone to weight gain. The treatment is levothyroxine, which is simply a replacement for the hormone your body is short of — not a stimulant or a "boost", but topping up what is missing. It works best taken on an empty stomach, well before food, and away from certain supplements such as iron and calcium that can block its absorption. For most people this is a long-term, often lifelong treatment, because the gland rarely recovers. Many feel much better once the right level is reached, though it can take a little while to settle.

An overactive thyroid: calming it down

In an overactive thyroid (hyperthyroidism), the gland makes too much hormone, which can cause a racing heart, weight loss, shakiness, anxiety and heat intolerance. Here the aim is the opposite — to reduce hormone production. Antithyroid drugs such as carbimazole work by slowing the gland's manufacture of thyroid hormone, gradually bringing levels back down. Sometimes a beta-blocker is added in the early weeks to ease symptoms such as a fast heart rate while the antithyroid medicine takes effect. Depending on the cause, treatment may be given for a defined period, or other options such as radioactive iodine or surgery may be considered. Your specialist will guide which path suits you.

How treatment is guided and monitored

Thyroid treatment is steered by blood tests rather than by symptoms alone, because how you feel does not always match what the gland is doing. These tests measure thyroid hormone levels and a pituitary signal called TSH, and the results tell your clinician whether to adjust your treatment up or down. Adjustments are made gradually, and you are rechecked after a settling-in period before any further change. Once you are stable, monitoring usually becomes much less frequent. It helps to keep to the same brand or preparation where possible, take your medicine consistently, and not adjust it yourself between tests, as steady, even treatment gives the most reliable readings.

The carbimazole warning and pregnancy notes

Carbimazole carries one rare but serious risk that everyone taking it must understand: it can occasionally cause a sudden drop in infection-fighting white blood cells, called agranulocytosis. The key warning signs are a sore throat, mouth ulcers, fever or feeling suddenly unwell — if these occur, stop and seek an urgent blood test the same day, because untreated it can be dangerous. This is not a reason to avoid carbimazole, but a reason to know the signal. Thyroid problems also need careful handling in pregnancy: both under- and overactive thyroid affect mother and baby, treatment is closely monitored, and the choice of antithyroid medicine may change, so always tell your clinician if you are pregnant or planning to be.