The thyroid gland: 7 facts worth knowing

The main points

The thyroid is small but reliable: it has its own hormone store, fine tuning for iodine and a "remote control" in the brain. In most people it copes for years, and a few simple habits help catch a problem early: test TSH at the same time of day, repeat a small abnormality after 2 to 3 months and tell your doctor about your thyroid before a contrast CT.

How to use these facts

If you have TSH tested, have blood drawn at about the same time of day: in the morning the value is on average higher. If the result is only slightly above normal, do not draw conclusions from one number: the recommendation is to repeat TSH with free T4 after 2 to 3 months. After a severe illness TSH can be temporarily raised, so a test right after recovery is also worth rechecking.

1. Size and blood flow

The thyroid of an adult without goitre weighs 6 to 20 g, depending on body size and iodine in the diet. At autopsy of 90 people aged 2 to 87, the mean weight in those over 15 was 14.2 ± 0.4 g (Zaichick V Sci Total Environ 1997. PMID 9373990^[14]). About 5 mL of blood per g of tissue flows through it every minute: with 0.4 per mille of body weight the gland takes 2% of all blood flow, which is 50 times its share by weight (our calculation); the whole blood volume passes through it about once an hour (Maenhaut C Endotext 2015. PMID 25905409^[1]).

2. A store of ready hormone

An adult body holds 15 to 20 mg of iodine, 70 to 80% of it in the thyroid; hormones are kept in the follicles as colloid for several months (Ahad F Indian J Endocrinol Metab 2010. PMID 21448409^[2]). The pool of organic iodine turns over by about 1% a day, which gives a time of the order of 100 days (our recalculation) (Rousset B Endotext 2015. PMID 25905405^[3]). In inflammation of the gland, cells break down and ready hormone is released into the blood: first a temporary excess, then often a temporary shortage, usually with recovery (De Leo S Lancet 2016. PMID 27038492^[15]).

3. The iodine pump and CT contrast

On each pass of blood the gland takes up about 20% of the iodide and holds 20 to 50 times more inside than in plasma; in Graves disease more than 100 times. Excess iodine briefly slows hormone production (the Wolff–Chaikoff effect) (Rousset B Endotext 2015. PMID 25905405^[3]).

An ordinary contrast CT supplies 2500 to 5000 µg of free iodine against a daily need of 150 µg, that is 17 to 33 times more (our calculation) (Lee S Y J Clin Endocrinol Metab 2015. PMID 25375985^[16]). In 422 people aged 50 to 65, with blood taken before and 4 to 12 weeks after a coronary CT, no one developed overt hypo- or hyperthyroidism; mild shifts or isolated changes in free T4 appeared in 3.5% of those whose hormones were normal before the CT; on the questionnaire, completed by 368 people, complaints did not increase. The risk was higher with antibodies to thyroid peroxidase; nodules did not raise it (Carlqvist J Eur Thyroid J 2024. PMID 39400593^[4]).

4. From precursor to working hormone

The main product of the thyroid is T4, a prohormone with four iodine atoms; the working T3 has three. In a day a healthy gland releases 94 to 110 µg of T4 and 10 to 22 µg of T3 (Rousset B Endotext 2015. PMID 25905405^[3]). About 80% of the T3 in blood is made outside the gland from T4, in the liver and kidneys (Mariotti S Endotext 2021. PMID 25905193^[5]); this needs the deiodinase enzymes, all of which contain selenium (Mullur R Physiol Rev 2014. PMID 24692351^[6]). But extra selenium beyond the diet, in a 5-year trial of 491 people aged 60 to 74 with borderline selenium, barely changed thyroid hormones, and the authors found no justification for taking it (361 completed) (Winther K H Eur J Endocrinol 2015. PMID 25740851^[17]).

5. The hormone works almost everywhere

The thyroid hormone T3 changes gene activity through nuclear receptors in practically every cell (Gereben B Endocr Rev 2008. PMID 18815314^[7]). That is why complaints in hypothyroidism vary: fatigue, lethargy, cold intolerance, weight gain, constipation, voice change, dry skin; total cholesterol and LDL cholesterol rise (Chaker L Lancet 2017. PMID 28336049^[8]). In older people an excess of hormone gives fewer symptoms but more heart complications (De Leo S Lancet 2016. PMID 27038492^[15]).

Low TSH in people over 60 is linked to atrial fibrillation: within 10 years it appeared in 28% with TSH of 0.1 mIU/L or lower versus 11% with normal TSH, hazard ratio 3.1 (95% CI 1.7 to 5.5). The study included 2007 people, the low-TSH group had 61 people, and many were taking thyroid hormone (Sawin C T N Engl J Med 1994. PMID 7935681^[18]).

6. The remote control in the brain: TSH

The pituitary controls the gland through TSH. The normal value in a healthy adult is about 0.4 to 4.0 mIU/L (sometimes up to 5.0); in primary hypothyroidism it is above 5 and up to hundreds, in hormone excess below 0.004 (Mariotti S Endotext 2021. PMID 25905193^[5]). TSH works like an amplifier: in mild hypothyroidism, when total T4 is still 77 to 116 nmol/L (6 to 9 µg/dL), TSH is already 5.3 mIU/L versus 1.3 in healthy people, 4 times higher; in severe hypothyroidism it is 149 mIU/L (115 times higher; our calculation), while T3 hardly changes until a severe stage (Mariotti S Endotext 2021. PMID 25905193^[5]).

TSH has a daily rhythm: a low in the early afternoon, about 30% higher in the evening and at night, with a peak between 9 pm and 2 am. Each person has their own "corridor" of TSH about a third of the width of the normal range (Pearce S H S Eur Thyroid J 2013. PMID 24783053^[9]). In one laboratory, among 20,019 samples, TSH above 4.0 mIU/L was found in 14.1% of samples at 7 to 8 am and 7.9% of samples at 2 to 3 pm; these are different people at different times, not one person over a day (Andersen I B PLoS One 2015. PMID 26461522^[10]).

7. Women are affected more often, and nodules are common

In a British 20-year follow-up of 2779 adults, hypothyroidism appeared in women at 3.5 per 1000 a year and in men at 0.6, so about 6 times less often (our calculation); hyperthyroidism 0.8 per 1000 a year in women and only isolated cases in men; 1877 survivors took part in the repeat (Vanderpump M P Clin Endocrinol 1995. PMID 7641412^[11]). In a Brazilian cohort of 9705 people over 4 years overt hypothyroidism was 3.2 times more frequent in women (0.76% a year versus 0.24%), all disorders together 1.36 times (Benseñor I M Arch Endocrinol Metab 2021. PMID 33844894^[12]). Hypothyroidism is also more frequent in people over 65 (Chaker L Lancet 2017. PMID 28336049^[8]).

Nodules: they can be felt in about 5% of women and 1% of men, and ultrasound finds them in 19 to 68% of randomly selected people; cancer is found in 7 to 15% of nodules. Usually nodules larger than 1 cm are assessed, and the signals for examination are rapid growth and hoarseness (Haugen B R Thyroid 2016. PMID 26462967^[13]).

Who should see a doctor first

Rapid growth of a lump on the neck, hoarseness, pain at the front of the neck, trouble swallowing: see a doctor. Fatigue, cold intolerance, constipation and weight gain with raised TSH, or palpitations and irregular beats in a person over 60 with low TSH: discuss the test results with your doctor. People with known untreated hormone excess are usually not given contrast for a CT.

Summary

The thyroid is built reliably: with a hormone store, fine tuning for iodine and TSH as the remote control. Test TSH at the same time of day, do not draw conclusions from a single number and see a doctor promptly if a lump, hoarseness or persistent complaints appear.