The simple thyroid test?

The simple thyroid function test is the most commonly requested endocrine investigation in general practice. And when you do get an abnormal result that fits nicely into the box of hypo or hyperthyroidism, it’s very satisfying to be able to diagnose and treat a condition that can make patients feel so dreadful. 

If only the human body read the guidelines from NICE, updated in October 2023, then that repeat test would behave beautifully and we would wave our happy patient off into the sunset. Except it doesn’t work like that. Thyroid problems are one of those areas I read up on, “get”, then seem to forget about a week later and find myself in a flap. This blog is a pocket-sized top up of helpful tips to do with everything thyroid related! 

1. Don’t check a patient's TFTs when they’re unwell (unless you think the acute symptom is due to thyroid disease). You won’t get an accurate result and interpretation is tricky. 
2. Beware patients with autoimmune diseases, such as T1DM and coeliac disease. They should have their TFTs checked at diagnosis and annually. Equally, if you diagnose someone with autoimmune thyroid disease, check them for diabetes and coeliac disease. 
3. New AF diagnosis? Check their TFTs. 
4. Wondering where to start with doses for hypothyroidism? Worry no more - 1.6mcg/kg/day (rounded to nearest 25mcg) for adults <65 years old with no CVD. For those >65 years old or with CVD, start with 25-50mcg & titrate cautiously.
5. If patients are on treatment for hypothyroidism, aim for a TSH within the normal range checking every 3 months and adjusting dose until stable, then check annually. 
6. When the TSH doesn’t behave, do the following: check bodyweight and recalculate dose based on the 1.6mcg/kg/day as above. Tell them to stop having a soya milk coffee with a papaya milkshake for breakfast – all of these ingredients impair absorption of levothyroxine. It should be taken on an empty stomach, with water, at least 30 minutes before breakfast or at bedtime. 
7. It's those tricky medications. Antacids, iron, PPIs, H2Rs, carbamazepine and phenytoin all mess with levothyroxine absorption or requirements. Always ask about over the counter medications and supplements too! Biotin supplements will affect TFT results- stop them then repeat. 
8. Weird bloods results? Normal TSH but high T3 & T4? Could it be Thyroid Hormone Resistance (THR)? An inherited disorder most commonly due to a mutation in the thyroid hormone receptor Beta, this affects around 1 in 40,000 patients. There may be symptoms of both hyper and hypothyroidism in different organ systems, making the diagnosis challenging and only able to be confirmed with genetic testing. If the blood results don’t make sense, or your patient isn’t improving, refer to endocrinology. 
9. In patients with hyperthyroidism, consider starting carbimazole and/or beta blockers while waiting for specialist review. Check for local pathways or use advice and guidance if you’re unsure. Unlike in hypothyroidism, hyperthyroid patients need more frequent monitoring. Every 6 weeks until TSH normalises, then every 3 months from then on. 
10. And finally, top tip number 10. Ideally, women of child-bearing age with any type of thyroid disease should receive pre-conception counselling and an early referral if they become pregnant. If you do find a woman with hypothyroidism who is pregnant, make sure she immediately increases her levothyroxine dose by 25mcg daily in women on ≤100mcg daily, or by 50 mcg daily in women on a dose of > 100mcg daily. 

Phew! A whistle stop tour through all things thyroid! Remember for more on this topic and everything else you need to know about hot topics in general practice, come to one of our new Spring 2024 NB Medical GP Update courses or sign up for NB Plus to access all our courses and books for a full 12 months.