What does the research show about thyroid stimulating hormone (tsh)?

Thyroid stimulating hormone (TSH)

TSH is the main screening test for thyroid function. Published upper limits for adults run from 2.5 to 4.8 mIU/L, rise with age and differ in pregnancy. In pooled cohorts, TSH of 10 or above went with more heart disease and TSH below 0.1 with more fractures. Levothyroxine for mildly raised TSH did not improve symptoms in trials.

Checked 2026-11-13

Status: A blood test ordered by a clinician and also sold direct to consumers by some US laboratories. FDA classifies thyroid stimulating hormone test systems as class II devices [1]. The US Preventive Services Task Force concluded in 2015 that the evidence was insufficient to weigh the benefits and harms of screening nonpregnant adults who have no symptoms [2].
Measures: The amount of thyroid stimulating hormone in blood, a hormone made by the pituitary gland [1].
Also called: TSH, thyrotropin, thyroid stimulating hormone, thyroid-stimulating hormone, thyrotropic hormone, thyrotrophin, TSH 3rd generation, third generation TSH, sensitive TSH, ultrasensitive TSH, TSH with reflex to free T4
Tested for: thyroid check, underactive thyroid, overactive thyroid, fatigue, weight gain, hair loss, checking a thyroid medication dose, pregnancy planning

Strong several reviews agree · Moderate one review or consistent trials · Limited one or a few small studies · Untested no study in people, listed with what it rests on

Linked in research5

StrongLinked to more deaths at low TSH, with no rise in deaths at mildly raised TSH, across three pooled analyses of cohorts.
ModerateLinked to more coronary heart disease at TSH of 10 mIU/L or above, with no link at 4.5 to 6.9 mIU/L or inside the reference range, in pooled cohorts.
ModerateLinked to more atrial fibrillation at TSH below 0.45 mIU/L, with no link inside the reference range or at mildly raised TSH, in pooled cohorts.
ModerateLinked to more hip and other fractures at TSH below 0.45 mIU/L, most at TSH below 0.10, with no link at raised TSH.
ModerateLinked to later hypothyroidism in a graded way, starting inside the reference range and rising further when thyroid antibodies are present.

Reported risks0

No risks sourced yet.

Reference ranges

Each range is the one its source published, in mIU/L. Where sources differ, each is shown.

American Thyroid Association and American Association of Clinical Endocrinologists hypothyroidism guideline, 2012

Nonpregnant adults in iodine-sufficient areas, when the laboratory has no reference range of its own · [3], [4]

Range	What the source calls it
below 0.45 mIU/L	No name given
0.45 to 4.12 mIU/L	"normal range"
above 4.12 mIU/L	No name given

The guideline says a laboratory's own reference range comes first and that the normal range changes with age. Where no such range is available it gives 4.12 mIU/L as the upper limit of normal and 0.45 mIU/L as the lower limit. The 4.12 figure comes from the NHANES III reference population: people with no reported thyroid disease and no thyroid antibodies who were not pregnant and not taking estrogens, androgens or lithium [3][4]. The guideline's own table lists other published upper limits, including 2.5 from the National Academy of Clinical Biochemistry and 4.5 for the NHANES III population screened only for reported thyroid disease. It reports 97.5th percentile values from 3.24 mIU/L in African Americans aged 30 to 39 to 7.84 in Mexican Americans aged 80 and older [3]. Lab reports also print this unit as µIU/mL or mU/L, which give the same number [5].

US Preventive Services Task Force recommendation statement, 2015

Nonpregnant adults with a normal thyroxine (T4) level · [2]

Range	What the source calls it
below 0.1 mIU/L	"clearly low or undetectable"
0.1 to 0.4 mIU/L	"low but detectable"
0.4 to 4.5 mIU/L	"reference interval"
4.5 to 10 mIU/L	"subclinical hypothyroidism"
above 10 mIU/L	"subclinical hypothyroidism"

The Task Force describes the 4.5 mIU/L upper threshold as commonly but arbitrarily defined, and splits subclinical hypothyroidism into 4.5 to 10.0 mIU/L and greater than 10.0 mIU/L. The two low bands are its two classes of subclinical hyperthyroidism. All of these terms apply only when T4 is normal. With an abnormal T4 the Task Force uses the terms overt hypothyroidism and overt hyperthyroidism. It prints the low band as about 0.1 to 0.4 mIU/L and the usual laboratory reference interval as about 0.4 to 4.5 mIU/L, so the two meet at 0.4. It states that TSH can vary by as much as 50% of mean values from day to day, that multiple tests over 3 to 6 months are used to confirm an abnormal finding, and that about 37% of people with subclinical hypothyroidism return to normal thyroid function without treatment after several years [2].

European Thyroid Association guidelines, 2013 and 2015

Nonpregnant adults whose free thyroid hormone levels are inside their reference intervals · [6], [7]

Range	What the source calls it
below 0.1 mIU/L	"grade 2 subclinical hyperthyroidism"
0.1 to 0.39 mIU/L	"grade 1 subclinical hyperthyroidism"
0.4 to 4 mIU/L	No name given
4 to 10 mIU/L	"mildly increased"
above 10 mIU/L	"more severely increased" One to see a doctor about

Two guidelines from one body are joined here so that the whole number line is covered. The 2015 guideline grades subclinical hyperthyroidism as grade 1 at 0.1 to 0.39 mIU/L and grade 2 below 0.1 mIU/L [7]. The 2013 guideline sorts subclinical hypothyroidism into mildly increased TSH at 4.0 to 10.0 mU/L and more severely increased TSH above 10 mU/L, and prints the lower half of the reference range as 0.4 to 2.5 mU/L. It gives no name to results between 0.4 and 4.0 [6]. These terms require free thyroid hormone levels inside their own reference intervals and a result that persists. The 2013 guideline calls for a repeat TSH and free T4 after 2 to 3 months, and for age-specific local ranges to be considered in older people [6].

American Board of Internal Medicine laboratory reference ranges, 2026

Adults, as a general laboratory reference range · [5]

Range	What the source calls it
below 0.5 mIU/L	No name given
0.5 to 4 mIU/L	"reference range"
above 4 mIU/L	No name given

Printed as 0.5 to 4.0 µU/mL (0.5 to 4.0 mU/L). The table names no assay, age group or pregnancy status [5].

US National Library of Medicine, MedlinePlus

Adults · [8]

Range	What the source calls it
below 0.4 mIU/L	No name given
0.4 to 4.8 mIU/L	"normal"
above 4.8 mIU/L	No name given

MedlinePlus states that experts do not fully agree on the upper number, that some laboratories use a higher upper limit for older people, even as high as 7 µU/mL, and that the range is different in pregnancy. It also states that TSH values vary during the day [8].

Proposed narrower upper limit, Wartofsky and Dickey, 2005

Adults, upper limit only · [9], [3]

Range	What the source calls it
2.5 or below mIU/L	No name given
above 2.5 mIU/L	No name given

A proposal, and one side of a published debate. The authors argued that earlier reference populations included people with unrecognized thyroid dysfunction, and cited National Academy of Clinical Biochemistry guidelines indicating that more than 95% of normal individuals have TSH below 2.5 mU/L [9]. A US guideline describes this as a suggestion to lower the upper limit of the reference range to 2.5 mIU/L and did not adopt it [3]. In a reply in the same journal, Surks and colleagues argued for keeping 4.5 mIU/L. In the NHANES III reference group, 85% had TSH below 2.5 mIU/L and a further 9.7%, about 20.6 million Americans, had TSH between 2.5 and 4.5 mIU/L, many of them without thyroid disease [10]. A later pooled analysis of 55,412 people found no difference in coronary heart disease between TSH of 3.50 to 4.49 and 0.45 to 1.49 mIU/L [11]. The proposal gives no lower limit, so this block does not place low results.

Thyroid Epidemiology, Audit, and Research Study, Scotland, 2013, ages 31 to 40

Adults aged 31 to 40 with no evidence of thyroid disease · [12]

Range	What the source calls it
below 0.51 mIU/L	No name given
0.51 to 3.98 mIU/L	"reference interval"
above 3.98 mIU/L	No name given

The 2.5th and 97.5th centiles of the latest TSH result in 153,127 adults in Tayside with no thyroid prescription, thyroid admission or surgery, radioactive iodine treatment or positive thyroid antibodies. Median TSH was 1.58 mU/L in this age group. The abstract prints centiles for the youngest and oldest age groups only [12]. In the US NHANES III survey, the 97.5th centile was 3.56 mIU/L at ages 20 to 29 [13].

Thyroid Epidemiology, Audit, and Research Study, Scotland, 2013, over age 90

Adults over 90 with no evidence of thyroid disease · [12]

Range	What the source calls it
below 0.31 mIU/L	No name given
0.31 to 5.94 mIU/L	"reference interval"
above 5.94 mIU/L	No name given

From the same 153,127 adults. Median TSH rose from 1.58 mU/L at ages 31 to 40 to 1.86 mU/L over age 90, the 97.5th centile rose from 3.98 to 5.94 mU/L and the 2.5th centile fell from 0.51 to 0.31 mU/L [12]. In the US NHANES III survey, the 97.5th centile was 7.49 mIU/L at ages 80 and older, and 70% of older people with TSH above 4.5 mIU/L were inside their age-specific range [13].

American Thyroid Association pregnancy guideline, 2017

Pregnancy from weeks 7 to 12 onward, when no pregnancy-specific range from the local or a similar population is available · [14]

Range	What the source calls it
4 or below mIU/L	No name given
above 4 mIU/L	No name given

The guideline's first choice is a trimester-specific range from the local population, set in pregnant women with no thyroid disease, no TPO antibodies and adequate iodine intake. Failing that, it describes lowering the nonpregnant lower limit by about 0.4 mU/L and the upper limit by about 0.5 mU/L in the first trimester. For the typical patient that gives an upper reference limit of 4.0 mU/L from the late first trimester, with a gradual return toward the nonpregnant range later in pregnancy. It prints no number for the lower limit, so this block does not place low results. It reports TSH below 0.4 mU/L in as many as 15% of women without thyroid disease in the first trimester [14].

Endocrine Society pregnancy guideline, 2012, first trimester

First trimester of pregnancy · [15]

Range	What the source calls it
below 0.1 mIU/L	No name given
0.1 to 2.5 mIU/L	"normal"
above 2.5 mIU/L	No name given

The guideline states that in the first trimester the normal range is reduced to 0.1 to 2.5 mIU/L, and that a result inside the classical 0.4 to 4.0 mIU/L range might be misread as normal. For women taking thyroxine it sets treatment targets of below 2.5 mIU/L in the first trimester and below 3 mIU/L in the second and third [15]. These limits are lower than the 4.0 mU/L of the 2017 American Thyroid Association guideline [14].

American Thyroid Association and American Association of Clinical Endocrinologists hypothyroidism guideline, 2012, first trimester

First trimester of pregnancy, when the laboratory has no trimester-specific range, upper limit only · [3]

Range	What the source calls it
2.5 or below mIU/L	No name given
above 2.5 mIU/L	No name given

The guideline gives upper limits only, so this block does not place low results. Its first choice is the laboratory's own trimester-specific range [3]. The 2017 American Thyroid Association pregnancy guideline moved to an upper limit of about 4.0 mU/L [14].

American Thyroid Association and American Association of Clinical Endocrinologists hypothyroidism guideline, 2012, second trimester

Second trimester of pregnancy, when the laboratory has no trimester-specific range, upper limit only · [3]

Range	What the source calls it
3 or below mIU/L	No name given
above 3 mIU/L	No name given

The guideline gives upper limits only, so this block does not place low results. Its first choice is the laboratory's own trimester-specific range [3].

American Thyroid Association and American Association of Clinical Endocrinologists hypothyroidism guideline, 2012, third trimester

Third trimester of pregnancy, when the laboratory has no trimester-specific range, upper limit only · [3]

Range	What the source calls it
3.5 or below mIU/L	No name given
above 3.5 mIU/L	No name given

The guideline gives upper limits only, so this block does not place low results. Its first choice is the laboratory's own trimester-specific range [3].

What has been shown to move it

Shown to lower it3

LevothyroxineStrong

Symptoms and quality of life in subclinical hypothyroidism

Not shown to improve quality of life or thyroid symptoms in adults with subclinical hypothyroidism, including those 65 or older, though it brought TSH into range.

The numbers and sources

Effect: Thyroid hormone lowered mean TSH into the reference range (0.5 to 3.7 mIU/L against 4.6 to 14.7 on placebo) but did not change general quality of life (SMD -0.11, 95% CI -0.25 to 0.03) or thyroid-related symptoms (SMD 0.01, 95% CI -0.12 to 0.14) [1]. In adults aged 65 or older, the change in Hypothyroid Symptoms score at 1 year was 0.2 on both levothyroxine and placebo (difference 0.0, 95% CI -2.0 to 2.1). Tiredness scores did not differ (difference 0.4, 95% CI -2.1 to 2.9), and no secondary outcome improved [2].
Population: 21 RCTs with 2,192 nonpregnant adults, 3 to 18 months [1]; 737 adults with mean age 74.4 and TSH 4.60 to 19.99 mIU/L [2].
Sources: [1], [2]
Caveats: Most participants had mildly raised TSH. [1] rated the evidence moderate to high quality. [2] was funded by the European Union FP7 and others.

IodineModerateStudied in pregnancy

Thyroid function in pregnancy

Mixed results: reviews agree iodine lowered maternal thyroglobulin in pregnancy, but disagree on whether TSH changed.

The numbers and sources

Effect: Most studies found no effect on maternal or infant TSH or free thyroxine. Most RCTs found lower maternal thyroglobulin, and 3 found less growth in thyroid volume [3]. Cochrane found no clear difference in hypothyroidism in pregnancy or postpartum and postpartum hyperthyroidism fell (RR 0.32, 95% CI 0.11 to 0.91, 3 trials, 543 women) [5]. A 2026 meta-analysis found lower TSH (SMD -0.14, 95% CI -0.23 to -0.05) and lower thyroglobulin (SMD -0.22, 95% CI -0.32 to -0.13) with supplementation, with larger TSH drops at 200 to 300 µg a day [6].
Population: 37 publications in mildly to moderately deficient women [3]; 14 trials, 11 contributing data from over 2,700 women [5]; iodine-deficient pregnant women in English and Chinese studies [6].
Sources: [3], [5], [6]
Caveats: Cochrane rated the evidence low or very low quality and found the data insufficient for meaningful conclusions [5]. The reviews agree on lower thyroglobulin and differ on TSH [3][6]. The 2026 abstract does not give the number of studies or women [6].

SeleniumModerate

Hashimoto's thyroiditis

Mixed results: thyroid antibodies fell in people with Hashimoto's in both reviews, while reviews disagree on whether TSH changed.

On TSH, sources disagree.

The numbers and sources

Effect: Thyroid peroxidase antibodies fell in both reviews. In people on levothyroxine, the weighted mean difference after 3 months was -271 (95% CI -366 to -175) [5]. Across 29 cohorts, the SMD was -0.96 (95% CI -1.36 to -0.56) [6]. On TSH, sources disagree. In people not on thyroid hormone, a 2017 review found no change in TSH, quality of life or ultrasound findings [7]. A 2024 review found TSH lower (SMD -0.21, 95% CI -0.43 to -0.02, 869 people) [6].
Population: Adults with chronic autoimmune thyroiditis, 16 trials [5], 9 trials [7] and 35 studies [6].
Sources: [5], [6], [7]
Caveats: Antibody levels are a lab marker. The 2016 review rated evidence quality low [5], and the 2024 review moderate, with I² of 90% for antibodies [6].

Shown to raise it2

Soy foodsModerateHow often is not known

Thyroid function

Shown to raise TSH slightly, by 0.25 mIU/L, in pooled trials, with no change in free T3 or free T4.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: Soy raised thyroid stimulating hormone (WMD 0.248 mIU/L, 95% CI 0.001 to 0.494, I² 80.31%). Free T3 (0.027 pmol/L, 95% CI -0.052 to 0.107) and free T4 (-0.003 pmol/L, 95% CI -0.018 to 0.011) did not change. The authors call the clinical significance of the TSH rise unclear [16].
Population: Randomized trials in 18 articles [16].
Sources: [16]
Caveats: High heterogeneity, and the lower bound of the TSH interval is close to zero [16].

CalciumLimitedVery common

Reduced levothyroxine absorption

Limited evidence: in 20 adults on levothyroxine, 1,200 mg of calcium carbonate lowered free T4 and raised TSH, which returned after stopping calcium.

The numbers and sources

How common: Very common: more than 1 in 10 people.
Effect: Taking 1,200 mg of calcium as carbonate with levothyroxine for 3 months lowered free T4 from 17 to 15 pmol/L and raised TSH from 1.6 to 2.7 mIU/L. Both returned after calcium stopped. 20% of patients had TSH above the normal range on calcium [15].
Population: 20 adults with hypothyroidism on stable levothyroxine [15].
Sources: [15]
Caveats: A small uncontrolled cohort, with lab data showing T4 binds to calcium carbonate in acid [15].

Tested, and not shown to change it2

AshwagandhaModerate

Thyroid hormones

Not shown to change TSH or T3 in adults across 23 trials. T4 rose slightly, by 0.61 µg/dL.

The numbers and sources

Effect: Across 23 placebo-controlled trials, TSH and T3 did not change, and T4 rose by 0.61 µg/dL (p = 0.02) [6].
Population: 1,706 adults [6].
Sources: [6]
Caveats: The authors note heterogeneity and limited data on some endpoints [6].

Paleo dietLimitedStudied in womenHow often is not known

Iodine

One trial found urinary iodine fell by half within 6 months in postmenopausal women, while it held steady on a Nordic guideline diet.

FT4 and TSH did not differ, and FT3 was lower at 6 months.

The numbers and sources

How common: How often is not known: the sources give no rate.
Effect: Median 24-hour urinary iodine concentration fell from 71.0 to 36.0 μg/L at 6 months (p 0.001), and 24-hour iodine excretion from 134.0 to 77.0 μg/d. Levels did not change on the comparison diet. FT4 and TSH did not differ, and FT3 was lower at 6 months [8]. In a 4-week trial, iodine intake was 47.9 μg a day lower than on the Australian national guide diet (95% CI 16.5 to 79.2) [7].
Population: 70 postmenopausal women with overweight or obesity over 2 years [8]; 39 women over 4 weeks [7].
Sources: [7], [8]
Caveats: The diet excludes table salt and dairy, which the authors call two of the largest iodine sources [8]. Small trials in women.

Type in your own result

How it compares with the reference method

Compared with: The same person's result before taking biotin
Effect: After 7 days of 10 mg a day of biotin, interference appeared in 9 of 23 assays built on biotin and streptavidin (39%) and in none of 14 assays built without them. Four of 15 biotin-based sandwich immunoassays read falsely low and 5 of 8 biotin-based competitive immunoassays read falsely high. The 11 analytes included TSH, free T4, free T3, total T4 and total T3, measured on 4 diagnostic systems [33].
Population: 6 adults described as healthy, mean age 38 [33].
Sources: [33]
Caveats: Six people. The abstract does not list which TSH assays were affected, and many assays showed no interference [33].

Did not match the reference method1

Limited

Time of day

One large study found the upper limit of the TSH reference interval shifted with the hour of the blood draw, because TSH rises overnight.

The numbers and sources

Compared with: Samples drawn at other hours of the day
Effect: In 465,593 TSH measurements from people without thyroid disease, the reference interval varied significantly by hour of day, age, sex and ethnicity, and not by time of year. The lower limit stayed constant, while the upper limit rose from 6.45 to 7.55 mIU/L with age, mainly through a larger night-time surge [31]. A review of the reference range debate states that early morning values are greater than later ones, and that the pattern is accentuated by sleep deprivation, strenuous exercise and night or evening shift work [10].
Population: 465,593 TSH measurements from people aged 1 to 104, on a single immunoassay method [31].
Sources: [31], [10]
Caveats: A retrospective analysis of laboratory records from one health system and one assay. The abstract does not give the size of the difference between hours [31].

What studies tie it to

Effect: In a 20-year British follow-up, a raised TSH alone carried odds ratios for developing hypothyroidism of 8 in women (95% CI 3 to 20) and 44 in men (19 to 104). With thyroid antibodies as well, the odds ratios were 38 (22 to 65) and 173 (81 to 370). Risk rose with TSH values above 2 mU/L at the first survey [21]. In an Australian cohort followed 13 years, hypothyroidism at follow-up was present in 12.0% of antibody-positive women with baseline TSH of 2.5 mU/L or less, 55.2% with TSH between 2.5 and 4.0 mU/L and 85.7% with TSH above 4.0 mU/L [22]. Among 82 women with subclinical hypothyroidism, overt hypothyroidism developed within 10 years in 0% of those starting at 4 to 6 mU/L, 42.8% at above 6 to 12 mU/L and 76.9% above 12 mU/L [23].
Population: 2,779 adults in the Whickham Survey, with 96% of survivors followed up at 20 years [21]. 1,184 adults in the Busselton Health Surveys of 1981 and 1994 [22]. 82 women followed a mean 9.2 years [23].
Sources: [21], [22], [23]
Caveats: Observational. The Australian study defined hypothyroidism at follow-up as TSH above 4.0 mU/L or thyroxine treatment [22]. The 10-year figures come from 82 women at one clinic [23].

Mixed results1

Moderate

Pregnancy and child outcomes

Mixed results: raised TSH in pregnancy was linked to more preterm birth, while a levothyroxine trial found no difference in child IQ or pregnancy outcomes.

The numbers and sources

Effect: In pooled data on 47,045 pregnant women, preterm birth occurred in 6.1% of women with a raised TSH and normal free T4 against 5.0% of women with normal results (odds ratio 1.29, 95% CI 1.01 to 1.64). Each standard deviation higher TSH had an odds ratio of 1.04 (1.00 to 1.09) [27]. In a trial of 677 pregnant women with TSH of 4.00 mU/L or more and normal free T4, the median child IQ score was 97 with levothyroxine and 94 with placebo (P = 0.71), with no significant differences in other pregnancy or neonatal outcomes [28].
Population: 47,045 pregnant women in 19 cohorts, median 12.9 weeks at blood sampling [27]. 677 women randomized at a mean of 16.7 weeks of pregnancy [28].
Sources: [27], [28]
Caveats: The cohort finding is observational. The trial began treatment between 8 and 20 weeks of pregnancy, so it does not describe earlier treatment [28].

Checked, no link found1

Strong

Symptoms and quality of life in trials that lowered a mildly raised TSH

Not shown to improve quality of life or thyroid symptoms when levothyroxine lowered TSH into range in 21 trials of adults with subclinical hypothyroidism.

The numbers and sources

Effect: In a trial of 737 adults aged 65 and older with TSH of 4.60 to 19.99 mIU/L, levothyroxine left TSH at 3.63 mIU/L at 1 year against 5.48 on placebo. The Hypothyroid Symptoms score did not differ (between-group difference 0.0, 95% CI -2.0 to 2.1), nor did the Tiredness score (0.4, -2.1 to 2.9) [24]. Across 21 trials, thyroid hormone brought mean TSH to 0.5 to 3.7 mIU/L against 4.6 to 14.7 mIU/L on placebo, with no benefit for general quality of life (standardized mean difference -0.11, 95% CI -0.25 to 0.03) or thyroid-related symptoms (0.01, -0.12 to 0.14) [25]. A guideline panel that reviewed the same trials issued a strong recommendation against thyroid hormones for adults with subclinical hypothyroidism, and stated that it does not apply to women trying to become pregnant or to people with TSH above 20 mIU/L [26].
Population: 737 adults, mean age 74.4, mean baseline TSH 6.40 mIU/L [24]. 2,192 nonpregnant adults in 21 randomized trials lasting 3 to 18 months [25].
Sources: [24], [25], [26]
Caveats: A marker that tracks an outcome is not always one whose correction changes it. The pooled trials lasted 3 to 18 months. The guideline panel judged effects on cardiovascular events and deaths to be uncertain, measured in one trial with few events [26].

Sources

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