CPE Monthly: Thyroid Disease and Weight Management
By Alexandria Hardy, RDN, LDN
Today’s Dietitian
Vol. 24 No. 7 P. 42

Suggested CDR Performance Indicators 8.1.2, 8.1.5, 9.1.3
CPE Level 2

Take this course and earn 2 CEUs on our Continuing Education Learning Library

Weight and thyroid function have been associated for decades, but recent research questions whether the link is causal.

A hallmark of an improperly functioning thyroid is metabolic dysfunction, which can cause weight instability and changes to the resting metabolic rate (RMR).1-3 Hyperthyroidism incites a hypermetabolic state in the body that can lead to weight loss, while hypothyroidism has the opposite effect and lowers the basal metabolic rate (BMR), which can contribute to weight gain.2,4 The thyroid produces thyroid hormone, which exists in two forms: free and total thyroxine (T4), and free and total triiodothyronine (T3). Thyrotropin-releasing hormone (TRH) from the hypothalamus and thyroid-stimulating hormone (TSH) from the pituitary gland together regulate the release of thyroid hormone, making TRH and TSH essential to the mechanisms of hypo- and hyperthyroidism.5

This continuing education course examines the intersection of thyroid dysfunction and body weight to explore the influence these lifestyle factors have on each other.

Role of the Thyroid
The thyroid is a small endocrine gland located in the neck. A properly functioning thyroid gland works in tandem with the hypothalamus and pituitary gland. The hypothalamus releases TRH, which incites the release of TSH from the pituitary gland. This provokes the production of T3 and T4, completing the regulatory circuit known as the hypothalamic-pituitary-thyroid axis.5,6 T3 accounts for 15% of thyroid hormone, and T4 comprises the remaining 85%. Nearly all (99.8%) of the T3 and T4 is bound to proteins, while the remaining 0.2% is known as free T3 and T4, meaning it has the ability to bind to a receptor. Free T3 is the active form of thyroid hormone; T4 is converted to T3 via deiodinase enzymes.6

Thyroid Hormones and Weight
Thyroid hormones are responsible for many physiological responses in the body. They help adjust BMR, aid in heat generation in the adrenergic nervous response, and can trigger triglyceride breakdown, lipogenesis (ie, synthesis of fatty acids from nonlipids), and gluconeogenesis, or the synthesis of glucose from noncarbohydrates.7,8

The functionality of these hormones can impact weight, body composition, and BMI. Conversely, overall health status and dietary intake, body weight, and total energy expenditure also can impact the effectiveness of thyroid hormone secretion.9,10 Some research suggests that impaired thyroid function with normal negative feedback regulation could be responsible for increasing weight and BMI through changes in metabolic rate.11 For example, anorexia nervosa leads to less total and free T4 and a smaller amount of TSH released in response to TRH.10 Conversely, an individual can have a TSH value in the high range of normal with a BMI defined as overweight or obese and not have a thyroid disorder.6

Obesity (BMI ≥30 kg/m2) is significantly associated with the development of both primary and secondary hypothyroidism, but there’s no association between obesity and hyperthyroidism. This may be attributed to the inflammatory, chronic nature of obesity, which can alter the cellular expression and level of thyroid hormones.9

Adaptive thermogenesis—a metabolic process in which resting and nonresting energy expenditure are altered without a related change in an individual’s body composition—may be another way to explain unintended weight loss or gain during thyroid dysfunction. This process is thought to be influenced in part by thyroid hormones.1

Thyroid Function Throughout the Lifespan
Thyroid function changes as part of normal aging processes. Data from the National Health and Nutrition Examination Survey III illustrated an age-related increase in the upper threshold of normal for TSH, from 3.5 milli-international units (mIU)/mL in individuals aged 20 to 29 to 7.5 mIU/mL for those aged 80 and older. This change may be due to a correlation between decreased TSH and increased mortality in older adults.6

In children, adolescents, and adults who have obesity, there’s positive correlation between BMI and TSH levels near the top of the normal range. Another correlation commonly seen in adults with obesity is between elevated TSH and T3 and low free T4.11 During pregnancy, TSH levels in the upper normal range may require consult with an endocrinologist and treatment with levothyroxine, a synthetic version of T4 used to replace the hormone when levels are low.6 Elevated TSH may be analogous to a blood glucose value suggesting prediabetes, indicating that it’s important to monitor TSH levels but that hypothyroidism isn’t yet present.

With two-thirds of the US population clinically classified as falling into the overweight or obese BMI categories, diagnosing thyroid disease while assessing its impact on weight is challenging. Hypothyroidism affects 0.2% to 0.4% of the US population, with subclinical hypothyroidism rates much higher at 3.9% to 8.5%.6 About 1.2% of Americans have hyperthyroidism.12

Population screening hasn’t been shown to be effective in early detection of thyroid disease.13 Symptoms of hyper- or hypothyroidism (such as weight loss or gain) may be present in many adults in a population screening but aren’t necessarily indicative of thyroid dysfunction. The gold standard for diagnosing hyper- and hypothyroidism is by checking TSH and free T4 levels.6,14

Normal Thyroid Levels and Treatment
The normal range for TSH is 0.35 to 4.5 mIU/mL, but some researchers argue that 0.5 to 2.5 mIU/mL should be considered normal. Levothyroxine and other replacement therapies may be used in a broader or more defined range as a goal for TSH levels, depending on the patient’s TSH level upon diagnosis.6

Dosages of oral levothyroxine, which is used for treating primary hypothyroidism, vary depending on age, gender, symptom intensity, comorbidities, and TSH levels.13,15

More research is needed to determine the effect of levothyroxine on symptoms such as weight, as normalization of thyroid hormones doesn’t guarantee weight loss or gain. Research is also necessary to assess an appropriate length of treatment and potential side effects.16,17

Common Nonautoimmune Thyroid Diseases
Many types of thyroid disease exist, but the main focus of this continuing education course is nonautoimmune thyroid disease, also known as thyroid dysfunction. 9 This includes hypo- and hyperthyroidism, both of which involve changes in thyroid hormones and hormone synthesis.

Nonautoimmune thyroid disease doesn’t include Hashimoto’s thyroiditis or Graves’ disease, which are considered autoimmune diseases and are thought to have genetic etiologies.

Primary Hypothyroidism
Hypothyroidism is characterized by an imbalance between the hypothalamus, pituitary gland, and thyroid gland and decreased expression of regulatory hormones TRH and TSH.18 Hypothyroidism typically is associated with higher body weight and BMI due to symptoms such as a lower BMR and thermogenesis, which can lead to a decreased renal flow and hyaluronic acid, resulting in water retention. Other visible symptoms include bloating (typically due to slow movement of digestive material through the intestines, leading to chronic constipation) and facial edema, which can give the appearance of weight gain.7

Body weight and BMI categories aren’t an indication of health—fat mass isn’t always elevated in individuals with hypothyroidism.7 While hypothyroidism is one of the most common endocrine disorders, it shares much of its signs and symptoms with other chronic diseases, making diagnosis difficult without TSH and T4 testing.6 This is particularly true for older women, who may experience fewer symptoms and in whom primary hypothyroidism is more common.13,16

Subclinical Hypothyroidism
Subclinical hypothyroidism occurs when TSH levels are slightly higher than normal (4.6 to 8 mIU/mL) and free T4 is normal. Individuals with subclinical hypothyroidism may have milder versions of the symptoms seen in clinically diagnosed hypothyroidism.6 It’s estimated that 3% to 10% of individuals in the United States have subclinical hypothyroidism.19

Nonautoimmune Hyperthyroidism
The three main manifestations of nonautoimmune hyperthyroidism (NAH) are autonomic adenoma, hereditary NAH, and persistent sporadic congenital NAH.4 NAH is diagnosed by measuring TH levels in plasma; treatment typically involves pharmacological intervention with anti-thyroid drugs that suppress thyroid activity or surgery.20

All three forms of NAH are due to elevated thyroid hormone levels of T3 and T4, which send the body into a hypermetabolic state. Symptoms include faster gastrointestinal transit, weight loss, hyperactivity, protruding eyes, racing heart, and anxiety.20

Current research is limited and inconclusive regarding average BMI, weight at time of diagnosis, and alterations in weight posttreatment.

Lifestyle Factors and Thyroid Function
A review published in the International Journal of Molecular Sciences studied the impact of lifestyle factors, including diet and exercise, on normative functionality of the thyroid hormone. They observed that it was typical for a positive association to exist between BMI, TSH, and free T3, though free T4 and BMI had an inconsistent relationship. The researchers hypothesized that an alteration in TSH and thyroid hormone may cause an increase in BMI due to a potential link with adipose tissue, which is responsible for producing pituitary TSH.21

The authors concluded that there are many discrepancies in study design and observation techniques amongst the existing research and that more standardized methods need to be used to further solidify the association among diet, exercise, and thyroid hormone.21

BMI and Thyroid Function
A retrospective, observational study published in the International Journal of General Medicine examined BMI changes in 34 healthy adults—17 with hyperthyroidism and 17 with hypothyroidism—of which 28 were female. The goal of the study was to determine whether normalizing thyroid hormone levels impacted BMI.7

This study was conducted at a hospital in Spain, where 10% of the general population has a form of thyroid disease (9.1% has hypothyroidism and 0.8% has hyperthyroidism). Participants in the hyper- and hypothyroidism groups had different thyroid etiologies but similar BMIs, weights, and heights; more than one-half had an autoimmune thyroid disease. Baseline levels of free T4 and TSH were significantly different than final free T4 and TSH levels.7

Posttreatment, there were no statistically significant changes in weight or BMI in either group. Participants with hypothyroidism lost an average of 2.25 kg, with more severe hypothyroidism leading to less weight loss (<2 kg). Those with hyperthyroidism gained 2.92 kg, while neither group moved out of the BMI category in which they began the study.7

The Prevention of Obesity Using Novel Dietary Strategies (POUNDS LOST) was a two-year randomized clinical trial that measured the functionality of thyroid hormones in participants with overweight and obesity. The study assessed whether following a specialized eating plan would alter body weight, RMR, and thyroid hormone levels.3

The study participants included 811 men and women aged 30 to 70 with normal thyroid function. Participants were randomized into four dietary groups, all of which included a 750-kcal reduction per day from their current diets but differed in macronutrient composition; two were classified as low-fat and two as high-fat.22 Group one consumed 20% fat, 15% protein, and 65% carbohydrates; group two consumed 20% fat, 25% protein, and 55% carbohydrates; group three consumed 40% fat, 15% protein, and 45% carbohydrates; and group four consumed 40% fat, 25% protein, and 35% carbohydrates. Diet adherence was self-reported through five-day diet records (at baseline) and 24-hour recalls at months six and 24.23 Body weight, waist circumference, RMR, TSH, and total and free T3 and T4 were measured at baseline and repeatedly throughout the study, and BMI was assessed at baseline and as weight changed. Eighty percent of the participants completed the POUNDS LOST trial, and there was no significant difference in weight loss between any of the diet groups.3,23

In a data analysis of 569 of the POUNDS LOST participants that evaluated the link between thyroid hormone levels, body weight, and RMR, participants lost an average of 6.6 kg (14.6 lbs) in the first six months and regained an average of 2.7 kg (6 lbs) in the remaining 18 months. Weight regain was attributed to reduced RMR, which is regulated by thyroid hormones. Participants with higher baseline free T3 and free T4 experienced significantly greater weight loss. The researchers hypothesized that free T3 or T4 levels may increase RMR, thus promoting a hypermetabolic state and an increase in calories burned. Total body weight at baseline wasn’t significantly associated with fluctuations in thyroid hormones, and there were no differences in overall thyroid hormone levels or RMR between study groups.3

The trial would need to be repeated in a population with previously established thyroid dysfunction to determine whether similar changes in hormones and weight would occur. The POUNDS LOST study suggests that individualized, calorie-restricted diets may be more effective for weight loss among individuals with overweight and obesity who have elevated free T3 and T4 levels.3

Nutrition and Thyroid Function

Dietary Patterns
The question of which dietary pattern is most appropriate for patients with thyroid dysfunction and weight instability remains largely unanswered. Few studies have examined the intersection of thyroid hormones, diets, and weight.

Macronutrient Focused Diets
Changing the percentage of fat, carbohydrates, and protein is one of the most basic building blocks that make up a dietary pattern. Increasing or decreasing one or more of these macronutrients is a fairly common diet technique in research, yet its ability to alter metabolic and hormonal responses is seen primarily when overall calories also are reduced. Additional research is needed to confirm an ideal macronutrient balance to support thyroid health and hormone functionality.24 Ketogenic diets are commonly used by consumers as a way to stimulate weight loss and improve cognitive, cardiometabolic, and thyroid function, though their impact on many of these biomarkers, including T3 and T4, are virtually unknown. A high-carbohydrate, low-fat diet has had inconsistent effects on thyroid hormones, and there’s little data on the efficacy of these dietary patterns in improving thyroid function and BMI.25

Intermittent Fasting
Individuals who desire weight loss without counting calories or eliminating food groups explore the dietary pattern of intermittent fasting (IF). No plethora of data is available on IF, weight management, and thyroid dysfunction, but a small study of male athletes who underwent time-restricted feeding (TRF) yielded interesting results that may be extrapolated to a larger population.

The subjects were resistance-trained and consumed either three meals per day at 1 pm, 4 pm, and 8 pm (TRF cohort) and fasted the remaining 16 hours of the day or consumed their total calories at 8 am, 1 pm, and 8 pm. There was no calorie restriction or change in diet quality or quantity. Both groups were calorie and macronutrient matched and encouraged to adhere to a standardized resistance training program and their customized dietary pattern for the eight-week study duration. Many metrics were measured at baseline and study conclusion, including fat and muscle mass, T3, and TSH. All athletes had normative thyroid function at baseline.26

At the conclusion of the study, there was no significant change in TSH for either group, but fat mass and T3 both decreased in the TRF cohort. Muscle mass also was maintained in both groups. This may indicate that the body began to use fatty acids as energy at rest, although this hypothesis wasn’t specifically proven.

The researchers concluded that practicing IF alongside resistance training may be beneficial in specific populations, although a much larger and diverse sample size would need to be studied. Moreover, a focus on thyroid hormones and body composition would be necessary to determine whether IF is an appropriate dietary recommendation for patients with thyroid dysfunction.26

Essential Nutrients
Many micronutrients are necessary to keep the thyroid gland functioning normally. Copper, iron, and iodine aid thyroid hormone synthesis, while the conversion of T4 to T3 requires selenium and zinc. Insufficient intake of these micronutrients can lead to irregular regulation. The most commonly consumed sources of iodine are iodized table salt, vegetables, and seafood and sea vegetables.5

Deficiency is more commonly found in individuals with autoimmune thyroid diseases, such as Hashimoto’s thyroiditis and Graves’ disease, as opposed to those who experience primary hyper- or hypothyroidism.27 Because the thyroid serves as a vitamin D receptor site, it’s not uncommon for individuals with thyroid dysfunction to have some level of vitamin D deficiency.18,21

Vitamin D research with a focus on deficiency and weight management is needed to determine whether supplementation or an increase in dietary sources of vitamin D could impact weight loss or gain. Examining lab work for potential vitamin and nutrient deficiencies, providing MNT for weight management, and using supplements when appropriate may have the potential to stabilize thyroid functionality and/or weight.

Many products falsely claim to increase metabolism and promote weight loss. These claims often are appealing to individuals who believe they have gained weight because of thyroid disease.

Research shows roughly 30% of individuals with thyroid disease take a nutraceutical, 70% of which boast inaccurate ingredient lists and unsubstantiated claims. Nutraceuticals, which currently aren’t subject to regulatory standards, are described as foods or nutrients that may positively impact health. Some of the most common nutraceuticals, which generally are added to the diet in supplement form, associated with thyroid health are carnitine, melatonin, inositol, and resveratrol. Nearly all recent research has been conducted in animals with impact on weight not being a primary endpoint, making it difficult to determine the effects of these supplements on the thyroid.

More research is recommended to determine appropriate dosages and combinations and whether similar beneficial effects would also be seen via dietary sources.28

A general positive association exists between probiotic intake and normative thyroid function (the majority of existing research has been conducted in animal trials). Preliminary results suggest that a flourishing gut microbiota impacts how well the thyroid absorbs and uses micronutrients.

Examining the impact of specific strains of probiotics and micronutrient supplementation on human thyroid functionality could provide clinicians with a therapeutic approach to moderating thyroid health and weight. The probiotic strain Lactobacillaceae and Bifidobacterium are some of the most commonly studied strains, but further research is needed to determine appropriate dosage levels if used therapeutically.27

Study Limitations
It’s common in meta-analyses to notice a pattern of weight loss and subsequent regain in patients with hyperthyroidism. This pattern is repeated in reverse in patients with hypothyroidism who tend to gain weight, undergo thyroid hormone replacement therapy, and then lose weight.

There’s currently little clinical data on dietary interventions and their long-term effects on BMI once thyroid function is normalized. Controlled intervention studies are needed to determine whether there are other comorbidities that impact weight, natural body set point, and changes in lean and fat mass after treatment.29

Areas of Further Research
Well-designed, large scale clinical trials are needed to further examine the association between BMI and thyroid functionality, particularly as weight and thyroid function changes throughout the life span.9 Observational studies are common but don’t elucidate causation.3 In terms of cultural and ethnic relevance, race doesn’t appear to have a clinically significant impact on TSH ranges but further clarity is needed to determine why this is true.6 Involving RDs in future research also could be helpful to determine whether regular, focused counseling and behavioral support enhances normative thyroid function or weight loss.

Understanding the physiological impact of weight change is crucial, which suggests that more trials need to study resting and basal metabolic rates in addition to thyroid hormones and weight change.3 Changes in thyroid hormone often are associated with weight gain and weight loss, which has led to speculation that thyroid dysfunction is directly tied to BMI. This hypothesis warrants further long-term study to determine whether this is a causal relationship or an association.7

Putting It Into Practice
Dietitians can assess current dietary intake through lab analysis and food recalls to determine where potential deficiencies lie. This enables RDs to educate patients on foods and drinks that are rich in potential missing micronutrients and teach them how to incorporate appropriate probiotic strains into their daily meals as an alternative or in addition to supplements. These sessions could be easily incorporated into a meal prep workshop, cooking lesson, virtual visit, or grocery store tour.

Many individuals with thyroid dysfunction struggle to adequately manage their diagnosis. There’s a lack of individualized treatment and nutrition guidance, inadequate accurate health-literate resources, and little-to-no monitoring from their diagnosing clinician to ensure changes are being made and recommendations are being met.30 Dietitians are well-placed to aid their patients by providing MNT, allowing for regular follow-up and a patient-specific treatment plan. The POUNDS LOST trial indicates the short-term metabolic benefits of consuming a calorically reduced, personalized dietary plan.22 Offering regular MNT follow-ups and behavioral support through in-person or virtual sessions also may be helpful. However, general weight loss and a decrease in BMI isn’t sufficient to reverse or significantly reduce thyroid dysfunction long term, and additional research is necessary.

In addition to improving the patient/dietitian relationship by allowing more frequent and consistent contact, RDs also can empower patients to take an active interest in their treatment and quality of life.30 Developing a broader understanding of the way weight and metabolic dysfunction impact one another is a crucial step in treating individuals with thyroid dysfunction.

— Alexandria Hardy, RDN, LDN, is a consultant in corporate wellness and a freelance food and nutrition writer based in Pennsylvania.


Learning Objectives
After completing this continuing education course, nutrition professionals should be better able to:
1. Distinguish between hypo- and hyperthyroidism and discuss their physiological impact on weight management.
2. Evaluate how thyroid hormones affect weight and describe their roles in energy expenditure.
3. Assess three ways dietitians can improve patient outcomes by overcoming common barriers to managing thyroid dysfunction.


CPE Monthly Examination

1. In the POUNDS LOST trial, what baseline value was linked to a lower resting metabolic rate in the first six months?
a. Thyroid-stimulating hormone (TSH)
b. Free triiodothyronine (T3) and thyroxine (T4)
c. Total T3
d. Bound T4

2. For those looking to achieve weight loss, what dietary pattern may best support normative thyroid function?
a. Ketogenic diet
b. Intermittent fasting
c. High-carbohydrate, low-fat diet
d. Individualized reduced-calorie diet

3. Which micronutrient helps regulate hormone synthesis in the thyroid?
a. Iodine
b. Magnesium
c. Calcium
d. Sodium

4. What is a classic symptom of primary hypothyroidism?
a. Tremors
b. Sweating
c. Bloating
d. Dehydration

5. What happens to the basal metabolic rate in individuals with hyperthyroidism?
a. Increases
b. Stays the same
c. Decreases
d. Fluctuates

6. What is the gold standard for diagnosing thyroid disease?
a. Population screening
b. Weight gain or loss
c. Measuring T3
d. Assessing TSH and T4

7. In the retrospective, observational study published in the International Journal of General Medicine, what effect did normalizing thyroid levels have on BMI?
a. Increased a BMI category
b. Decreased a BMI category
c. Maintained baseline BMI category
d. Insufficient data to answer

8. What type of clinical trials are most needed to better understand the association between body weight and thyroid function?
a. Meta-analyses
b. Controlled intervention studies
c. Treatment trials
d. Cross-sectional studies

9. Which thyroid hormone increases throughout the lifespan?
a. T3
b. T4
c. Thyrotropin-releasing hormone
d. TSH

10. What is the percentage of individuals with subclinical hypothyroidism in the United States?
a. 3% to 10%
b. 10% to 17%
c. 17% to 24%
d. 24% to 31%


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