Thyroxine and thyroid stimulating hormone levels: New research

The thyroid gland produces hormones that regulate metabolism, in particular a hormone called thyroxine. Hypothyroidism occurs when the thyroid gland produces too little thyroxine. This condition affects approximately 3% of the UK population and is easily treated through taking daily doses of thyroxine hormone replacement. When treating hypothyroidism, clinicians aim to ensure that levels of another hormone called TSH also remains in the normal range. TSH stimulates the thyroid gland to release thyroxine. Its levels are controlled by thyroxine in a negative feedback loop, with greater levels of thyroxine leading to a decreased TSH production. There have been concerns that abnormal TSH levels can be bad for health and lead to a greater risk of a number of conditions, such as heart disease and osteoporosis.

Dr Graham Leese, from Ninewells Hospital and Medical School, led a research team examining patients on thyroxine replacement therapy. They studied how variations in these patients' TSH levels affected their long-term health. The study followed 16,426 patients taking thyroxine (86% female, mean age 60 years) and examined how their risk of contracting a range of diseases varied with their TSH levels.

The study found that patients with very high (>4.0mU/l) or suppressed (≤0.03mU/l) TSH levels more frequently suffered from heart disease, abnormal heartbeat patterns and bone fractures compared to patients whose TSH levels are in the normal range (0.4-4.0 mU/l). Patients who had a slightly low TSH level (0.04-0.4mU/l) did not have an increased risk of contracting any of these conditions.

These results show for the first time that it may be safe for patients taking long-term thyroxine replacement therapy to have a low but not suppressed TSH level. These patients do not show an increased risk of suffering from heart disease, bone fractures or abnormal heartbeat patterns. This means that patients may be able to take slightly higher doses of thyroxine than are currently recommended without having an adverse effect on their health.

Researcher Dr Graham Leese, a Consultant at NHS Tayside and Honorary Reader in Diabetes and Endocrinology at the University of Dundee, said:

"This is the first population-based study to show that having slightly lower TSH levels when taking thyroxine replacement is not detrimental to health. We found that having high or suppressed levels of TSH did lead to a higher risk of heart disease and bone fractures and so it is important that we continue to monitor TSH levels carefully to ensure that neither of these situations happen.

"Prescribing thyroid hormone replacement therapy is a careful balancing act as doctors need to ensure that the levels of a number of hormones all remain with the normal range after treatment. Our findings mean that it may be safe for patients with hypothyroidism to take marginally higher doses of thyroxine than are currently recommended. However careful monitoring of these patients would still required. We now need other studies to confirm our findings before any changes are made to routine clinical care."