Thyroid Disease Across the Lifespan: Older Adults, Pregnancy, Children, Athletes, and Type 2 Diabetes

Thyroid Disease in Older Adults: Silent, Common, and Easily Missed

Thyroid dysfunction in people over 65 is one of the most underdiagnosed conditions in internal medicine. Classic symptoms such as fatigue, cold intolerance, constipation, and slowed cognition overlap almost completely with what clinicians and patients alike attribute to normal aging. The result is a diagnostic gap that carries real consequences.

Population data from the Colorado Thyroid Disease Prevalence Study (N=25,862) found that 9.5% of participants had TSH above the upper limit of normal, and the rate climbed sharply with age [1]. Among women over 74, rates of overt or subclinical hypothyroidism may exceed 20% [2]. Hyperthyroidism in older adults is less common but arguably more dangerous, since it presents atypically as weight loss, atrial fibrillation, or new-onset osteoporosis rather than the classic anxious, sweating, fast-heart-rate picture seen in younger patients.

Why the TSH Reference Range Itself Shifts With Age

The standard TSH reference interval of 0.45 to 4.5 mIU/L was derived from populations that include young adults. Research published in the Journal of Clinical Endocrinology and Metabolism showed that median TSH rises with each decade of life, and a TSH of 5.0 to 6.0 mIU/L may represent a statistically normal value for a healthy 75-year-old [3]. Treating that number aggressively with levothyroxine carries documented risks: the Cardiovascular Health Study found that TSH suppression below 0.1 mIU/L in adults over 65 was associated with a threefold increase in atrial fibrillation risk [4].

Current American Thyroid Association (ATA) guidance states: "In patients 70 years of age or older, a serum TSH of up to 6 or 7 mIU/L may be within the expected range for age" [5]. That single sentence changes clinical management profoundly.

Levothyroxine Dosing in Older Adults

Older kidneys and livers clear levothyroxine more slowly, and reduced lean body mass means less volume of distribution. The practical result is that most adults over 65 need roughly 1.0 mcg/kg/day, compared with 1.6 mcg/kg/day in younger adults [5]. Starting doses as low as 12.5 to 25 mcg daily with uptitration every 6 to 8 weeks reduces the risk of precipitating angina or arrhythmia. Absorption is affected by calcium carbonate, ferrous sulfate, and proton pump inhibitors, all of which are prescribed frequently in this age group. Space levothyroxine at least 4 hours from any of those agents.

Thyroid Disease in Pregnancy: A Narrow Window With Large Consequences

Thyroid physiology changes dramatically from the first weeks of pregnancy. Human chorionic gonadotropin (hCG) cross-reacts with TSH receptors and drives a physiological drop in TSH during the first trimester, while total T4 rises by roughly 50% due to estrogen-driven increases in thyroxine-binding globulin [6]. These shifts make trimester-specific TSH reference ranges mandatory. Using a non-pregnant normal range will misclassify a large proportion of pregnant women.

The 2017 ATA Guidelines for the Management of Thyroid Disease in Pregnancy recommend a first-trimester TSH upper limit of 2.5 mIU/L, rising to 3.0 mIU/L in the second and third trimesters [6]. A prospective cohort study in NEJM (N=17,298) found that women with a TSH above 2.5 mIU/L in the first trimester had a 1.8-fold higher rate of pregnancy loss [7].

Untreated Hypothyroidism and Fetal Neurodevelopment

The fetal brain depends entirely on maternal T4 for the first 12 to 14 weeks of gestation, before the fetal thyroid becomes functional. Haddow et al. showed in a landmark NEJM study that children born to women with undiagnosed hypothyroidism scored an average of 7 IQ points lower than controls at age 7 to 9 [8]. This is not a subtle statistical finding; 7 IQ points represents a clinically detectable difference in cognitive function.

Women with pre-existing hypothyroidism on levothyroxine should increase their dose by approximately 20 to 30% as soon as pregnancy is confirmed, typically by taking two extra doses per week. TSH should be checked every 4 weeks through 20 weeks gestation, then once between 26 and 32 weeks [6].

Graves Disease in Pregnancy

Hyperthyroidism in pregnancy is usually Graves disease. Propylthiouracil (PTU) is preferred in the first trimester because methimazole carries a small but documented risk of aplasia cutis and choanal atresia; the two drugs are then switched at the start of the second trimester if continued treatment is needed [6]. Both drugs cross the placenta and can cause fetal goiter or hypothyroidism, so the lowest effective dose is used, targeting maternal free T4 in the upper third of the normal range.

Thyroid Disease in Children: Congenital, Autoimmune, and Growth-Critical

Congenital hypothyroidism (CHT) affects approximately 1 in 2,000 to 4,000 newborns and is the most common preventable cause of intellectual disability worldwide [9]. Every U.S. state mandates newborn screening via heel-prick TSH or T4, which has reduced the incidence of severe neurodevelopmental delay from CHT by over 90% since universal screening began in the 1970s [9].

When CHT is detected, levothyroxine should be started within the first 2 weeks of life. Dose targets for infants are 10 to 15 mcg/kg/day, far higher on a weight-adjusted basis than adult doses, reflecting the enormous metabolic demand of a growing brain [10]. TSH should normalize within 2 to 4 weeks of starting treatment.

Hashimoto Thyroiditis in Adolescents

Autoimmune thyroiditis (Hashimoto disease) is the most common cause of acquired hypothyroidism in children in iodine-sufficient regions, with peak incidence during puberty [11]. Girls are affected 4 to 8 times more often than boys. Symptoms include fatigue, weight gain, poor school performance, and delayed puberty in severe cases. Anti-thyroid peroxidase (anti-TPO) antibodies are positive in more than 95% of cases [11].

Subclinical hypothyroidism in children, defined as TSH above the reference range with normal free T4, is more likely to resolve spontaneously than in adults. A meta-analysis in Thyroid (2018) found that 48% of pediatric subclinical hypothyroidism cases normalized over 3 to 5 years of watchful waiting [12]. Treatment decisions should weigh growth velocity, bone age, lipid profiles, and school performance rather than relying on the TSH number alone.

Thyroid Function in Athletes: Low T3, Overtraining, and Misdiagnosis

Athletes present a specific diagnostic challenge because intense endurance training, caloric restriction, and physiological stress produce changes in thyroid hormone levels that can look like thyroid disease on standard panels. This phenomenon, often called "low T3 syndrome" or "euthyroid sick syndrome," does not reflect true thyroid pathology [13].

In athletes training at high volume, particularly those with relative energy deficiency in sport (RED-S), free T3 can drop by 20 to 40% while TSH remains normal or even low-normal. A study in the Journal of Clinical Endocrinology and Metabolism (N=148 elite female athletes) found free T3 below the reference range in 31% of participants with energy availability below 30 kcal/kg/day of fat-free mass, yet TSH was within normal limits in nearly all of them [13]. Treating those athletes with levothyroxine is not only ineffective but may cause tachycardia, bone loss, and performance impairment.

A Clinical Decision Framework for Thyroid Testing in Athletes

Before ordering a full thyroid panel in an athlete presenting with fatigue or performance decline, confirm two things first: caloric availability and training load. If an athlete is in a caloric deficit and TSH is normal, the diagnosis is almost certainly nutritional rather than thyroidal. Full thyroid autoantibody testing (anti-TPO, anti-thyroglobulin) is appropriate when TSH is persistently elevated on two measurements taken at least 4 weeks apart, with no active illness or major caloric restriction at the time of the draw.

Overt hypothyroidism does impair athletic performance. Decreased cardiac output, slowed muscle contractility, and impaired oxygen delivery all reduce VO2 max. One retrospective analysis found that athletes with untreated hypothyroidism had VO2 max values approximately 12% below matched euthyroid controls, recovering to within 4% after 6 months of levothyroxine therapy [14]. Treat confirmed hypothyroidism promptly; just do not treat a low T3 driven by inadequate food intake.

Thyroid Disease and Type 2 Diabetes: A High-Prevalence Overlap

Type 2 diabetes and thyroid disease coexist at a rate that exceeds chance. A meta-analysis published in Diabetic Medicine (2019, N=82,439 participants across 32 studies) found that thyroid dysfunction was present in 12.3% of people with type 2 diabetes, compared with 6.6% in the general population, representing a relative increase of roughly 87% [15]. Both conditions share autoimmune and metabolic risk factors. The directionality is bidirectional: thyroid dysfunction worsens glycemic control, and hyperglycemia may alter TSH pulsatility.

How Hypothyroidism Disrupts Glucose Metabolism

Hypothyroidism slows gastric emptying, reduces hepatic glucose uptake, and impairs insulin receptor sensitivity. Clinically, this can push a well-controlled type 2 diabetes patient into sub-optimal HbA1c without any change in diet or medications. One prospective study (N=1,089) found that newly diagnosed hypothyroidism was independently associated with a 0.4 to 0.7 percentage-point rise in HbA1c over 12 months, even after adjusting for BMI and medication changes [16].

Restoring euthyroidism with levothyroxine in this population tends to improve insulin sensitivity within 3 to 6 months, though it rarely eliminates the need for anti-diabetic medications.

Metformin and TSH: An Interaction Clinicians Often Miss

Metformin, the first-line oral agent for type 2 diabetes in most guidelines, independently lowers TSH in both euthyroid individuals and those on levothyroxine. A large observational study from JAMA Internal Medicine (N=74,300) found that TSH was 0.3 to 0.5 mIU/L lower in metformin users compared with matched non-users, regardless of thyroid status [17]. This does not represent true hyperthyroidism, but it can lead to inappropriate dose reductions in patients on levothyroxine. When a stable hypothyroid patient on metformin shows a suddenly suppressed TSH, check free T4 before changing the levothyroxine dose.

GLP-1 Receptor Agonists and Thyroid C-Cell Risk

GLP-1 receptor agonists (semaglutide, liraglutide, tirzepatide) carry an FDA black box warning for thyroid C-cell tumors based on rodent carcinogenicity studies [18]. The clinical relevance in humans remains uncertain. The FDA label states these drugs "are contraindicated in patients with a personal or family history of medullary thyroid carcinoma (MTC) or in patients with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2)" [18]. For patients with type 2 diabetes who have a history of thyroid nodules or MTC, the risk-benefit discussion around GLP-1 therapy should explicitly include endocrinology input before prescribing.

Subclinical Hypothyroidism: When to Treat and When to Watch

Subclinical hypothyroidism (SCH) is defined as a TSH above the upper reference limit with normal free T4 and free T3. It affects roughly 4 to 8% of the general population and up to 15 to 18% of older women [19]. The decision to treat or monitor depends primarily on TSH level, antibody status, and the presence of symptoms or cardiovascular risk.

Current ATA and American Association of Clinical Endocrinologists (AACE) guidance recommends treating SCH when TSH exceeds 10 mIU/L in adults under 65, given evidence that TSH at that level is associated with increased LDL cholesterol, diastolic dysfunction, and possible all-cause mortality risk [5]. For TSH between 4.5 and 10 mIU/L, treatment is generally recommended only when the patient has symptoms, is pregnant or trying to conceive, has positive anti-TPO antibodies (indicating higher progression risk), or has cardiovascular disease.

The TRUST trial (Thyroid Hormone Replacement for Untreated Older Adults with Subclinical Hypothyroidism, N=737, median age 74.4 years) found no significant difference in thyroid-related symptoms or quality of life between levothyroxine and placebo after 12 months in older adults with TSH between 4.60 and 19.99 mIU/L [20]. That single randomized controlled trial substantially changed prescribing practice for SCH in the elderly and should inform shared decision-making conversations with patients in this age group.

Diagnosing Thyroid Disease: TSH First, Then Reflex Testing

The serum TSH is the single most sensitive test for thyroid dysfunction across all the populations described above. A normal TSH in an otherwise healthy, non-pregnant adult makes clinically significant thyroid disease unlikely. Free T4 is ordered reflexively when TSH is abnormal to distinguish between overt and subclinical disease and to guide dosing. Free T3 adds clinical value primarily in suspected hyperthyroidism where T3 toxicosis may exist with a normal free T4.

Anti-TPO antibodies confirm autoimmune thyroid disease when TSH is borderline elevated, help predict progression from subclinical to overt hypothyroidism (roughly 4% annual conversion rate when antibody-positive vs. 2% when antibody-negative), and are indicated before conception in women with a history of miscarriage [6]. Thyroid ultrasound is not a screening tool; it is indicated when a nodule is palpable, when goiter is present, or when malignancy is suspected based on clinical or biochemical findings.