Thyroid storm is one of the most dangerous endocrine emergencies — the Burch-Wartofsky scoring system quantifies the risk: points are allocated for temperature (37–37.9°C = 5 pts; ≥40°C = 30 pts), tachycardia, AF, heart failure, CNS effects, and a precipitating event. A score ≥45 = thyroid storm; 25–44 = impending storm. The precipitants of thyroid storm are: thyroid surgery, radioiodine therapy, iodinated contrast agents, acute illness, trauma, or abrupt discontinuation of antithyroid drugs. The priority order of treatment: (1) propranolol (block peripheral effects of thyroid hormones — tachycardia, tremor, anxiety) first; (2) PTU or carbimazole (block new hormone synthesis) — PTU preferred in storm because it also blocks peripheral T4-to-T3 conversion; (3) hydrocortisone IV (blocks T4→T3 conversion + supports adrenal function, which is often depleted in severe thyrotoxicosis); (4) Lugol's iodine or potassium iodide (Wolff-Chaikoff effect — reduces thyroid hormone release) — MUST be given ≥1 hour AFTER PTU to prevent iodine being used as substrate for new hormone synthesis. The gestational hypothyroidism threshold is lower than in non-pregnant adults because TSH normally falls in the first trimester due to hCG cross-reactivity with TSH receptors — the upper limit of normal TSH in the first trimester is approximately 2.5–4.0 mIU/L (laboratory-specific), and fetal brain development depends entirely on maternal thyroid hormones in the first 12 weeks.

The subclinical hypothyroidism treatment threshold is one of the most debated areas in thyroid medicine — the evidence base for treating TSH between 5–10 mIU/L in the absence of symptoms and antibodies is limited. The TRUST trial (2017) — the largest RCT to date of levothyroxine in subclinical hypothyroidism in adults over 65 — showed no benefit on symptoms, quality of life, or any clinical outcome compared to placebo. However, the subset with TPO antibodies and the subset with TSH above 10 mIU/L were not well-represented in this trial, and multiple observational studies suggest these groups benefit from treatment. The current pragmatic approach: TSH >10 = treat (regardless of age, symptoms, or antibody status — cardiovascular risk and progression to overt hypothyroidism justify treatment); TSH 5–10 = treat if symptomatic OR if TPO antibodies positive; TSH 5–10 without symptoms and antibody-negative = monitor every 12 months without treatment. For patients already on levothyroxine with TSH in the 5–10 range, the dose should be titrated up — this is not 'subclinical hypothyroidism,' it is undertreated hypothyroidism.

TRAb (TSH receptor antibody) testing is the key diagnostic test for Graves' disease — it has approximately 95% sensitivity and 99% specificity for Graves' disease when measured with the newer third-generation assays. The clinical value: distinguishing Graves' disease (requires antithyroid drugs ± radioiodine/surgery) from thyroiditis (transient hyperthyroidism that resolves spontaneously — antithyroid drugs not needed) without requiring isotope scanning. In a patient with suppressed TSH + elevated FT4 + positive TRAb = Graves' disease diagnosis confirmed. If TRAb is negative in a thyrotoxic patient: consider thyroiditis (subacute De Quervain's — painful thyroid, raised ESR, often post-viral; postpartum thyroiditis — 4–12 months postpartum, no pain), TMNG (USS shows nodular gland), or toxic adenoma (USS shows single hot nodule). Amiodarone-induced thyroid dysfunction is a complex special case that warrants specific mention: amiodarone is 37% iodine by weight and causes thyroid dysfunction in approximately 15–20% of patients. Type 1 (iodine-induced excess synthesis — occurs in pre-existing goitre/Graves') and Type 2 (destructive thyroiditis from amiodarone toxicity) require different treatments and are distinguished by thyroid USS (vascularity — Type 1 hypervascular; Type 2 avascular). All patients on amiodarone should have TFTs every 6 months.

Pregnancy TFT interpretation requires specific laboratory ranges — the TSH reference range changes across each trimester due to hCG cross-stimulation of TSH receptors (hCG is structurally similar to TSH). First trimester: TSH typically 0.1–2.5 mIU/L (some laboratories use up to 4.0 mIU/L as the upper limit). Second trimester: 0.2–3.0 mIU/L. Third trimester: 0.3–3.5 mIU/L. GPs must use trimester-specific or pregnancy-specific reference ranges when interpreting TFTs in pregnant women — applying non-pregnant reference ranges will lead to over-treatment in the first trimester (where TSH below 0.5 is normal due to hCG stimulation) and under-treatment in the third trimester. The key safety rule: in the first trimester, any TSH above the upper limit for that trimester in a symptomatic woman, or above 2.5 mIU/L in a woman with TPO antibodies, should prompt levothyroxine initiation or dose increase (if already on levothyroxine) with target TSH <2.5 mIU/L.

The GP initiation of carbimazole for overt hyperthyroidism before endocrinology review is appropriate when waiting times are long — starting carbimazole promptly (rather than waiting weeks for an endocrinology appointment) reduces the risk of complications (AF, osteoporosis, cardiac failure from thyrotoxic cardiomyopathy). Carbimazole starting dose: 20–40 mg OD for overt hyperthyroidism (Graves' or TMNG). The GP who starts carbimazole should: (1) arrange TFT check at 4–6 weeks to assess response; (2) warn the patient about agranulocytosis (sore throat → stop carbimazole immediately, check FBC same day); (3) explain that carbimazole must be taken every day — missing doses allows rebound thyrotoxicosis; (4) add to medical record as a medication requiring weekly FBC monitoring if sore throat/fever develops. The agranulocytosis risk with carbimazole is approximately 0.3% (rare but serious) — it is dose-related and occurs most commonly in the first 3 months. Every patient starting carbimazole must receive written or verbal information: 'If you develop a sore throat or high temperature while taking this medication, stop it immediately and get a blood test the same day.'

The levothyroxine timing and absorption interaction is one of the most common causes of inadequate treatment response in hypothyroidism — levothyroxine is poorly absorbed when taken with food, calcium supplements, iron supplements, PPIs, and various other medications. The standard advice (take 30–60 minutes before breakfast on an empty stomach) achieves maximum absorption. However, patient adherence to this timing is poor in practice. The bedtime alternative (levothyroxine taken at bedtime, at least 4 hours after the last meal) has been shown in multiple RCTs to achieve equivalent or marginally superior TSH control compared to morning dosing, and may improve compliance by integrating the medication into the established bedtime routine. The key interactions: calcium carbonate reduces levothyroxine absorption by approximately 40% when co-administered — patients taking calcium supplements (for osteoporosis) should take levothyroxine and calcium at least 4 hours apart. Iron supplements similarly reduce absorption. PPIs moderately reduce levothyroxine absorption and may necessitate dose increases. Cholestyramine, colestipol, and sucralfate severely impair absorption.

The Graves' ophthalmopathy management involves two critical points that GPs should know: (1) selenium 200 mcg BD for 6 months is a NICE and European Thyroid Association recommendation for mild-moderate active Graves' ophthalmopathy — it reduces inflammation and disease progression; selenium is inexpensive, safe at this dose, and can be prescribed by GPs; and (2) radioiodine treatment for hyperthyroidism can transiently worsen Graves' eye disease — patients with moderate-severe ophthalmopathy should receive prophylactic oral prednisolone (0.4 mg/kg/day × 3 months, then tapering) when radioiodine is given, to prevent exacerbation. Patients with sight-threatening ophthalmopathy (corneal exposure, optic neuropathy) should have radioiodine deferred until the ophthalmopathy is stabilised or treated (IV methylprednisolone pulse therapy, orbital decompression surgery).

The autoimmune thyroid disease-coeliac disease association is clinically important — the prevalence of coeliac disease in patients with Hashimoto's thyroiditis is approximately 5–7% (10× the general population prevalence), reflecting the shared autoimmune genetic background (HLA-DQ2/DQ8). In practice, anti-tTG IgA + total IgA should be checked in all patients diagnosed with Hashimoto's thyroiditis, even in the absence of GI symptoms. The clinical implication: undiagnosed coeliac disease in a patient with Hashimoto's thyroiditis causes variable levothyroxine absorption (due to small intestinal malabsorption) + ongoing autoimmune stimulation. Starting a gluten-free diet in a patient with both conditions can: reduce anti-TPO antibody titres, improve levothyroxine absorption stability, and in some cases improve thyroid function test results. The polyglandular autoimmune syndrome type II (Schmidt's syndrome) association with Hashimoto's is the most important safety consideration: a patient with Hashimoto's who develops increasing fatigue despite optimal levothyroxine + dizziness + hyperpigmentation should be investigated urgently for Addison's disease (9am cortisol). Treating Addison's with cortisol replacement before levothyroxine dose increases is essential to prevent precipitating adrenal crisis.

The carbimazole-agranulocytosis education is a MHRA Black Triangle prescribing safety requirement — every patient starting carbimazole (or propylthiouracil) must receive the following information, documented in the clinical record: 'If you develop a sore throat, mouth ulcers, fever, or any sign of infection while taking this medication, stop taking it immediately and attend for a blood test the same day. Do not wait for a GP appointment — call the surgery and ask for an urgent FBC, or attend A&E out of hours.' This education must be repeated at each prescription renewal. GPs who prescribe carbimazole and do not document this safety education are at medico-legal risk if agranulocytosis occurs. The agranulocytosis is typically neutropenic (ANC <0.5 × 10⁹/L), develops rapidly (within weeks of a dose change in most cases), and can cause fatal sepsis from routine mouth bacteria within 48 hours of symptom onset. Stopping carbimazole immediately at the first symptom of infection and checking FBC within hours is the only safe response.