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Review Article
The Influence of Thyroid Dysfunction on Body Composition and 
Weight Trajectory
Jacqueline Jonklaas, MD, PhD *
Division of Endocrinology, Georgetown University, Washington, District of Columbia
a r t i c l e i n f o
Article history:
Received 24 July 2025
Received in revised form
13 November 2025
Accepted 25 November 2025 
Available online 18 December 2025
Key words: 
energy balance 
hyperthyroidism 
hypothyroidism 
thyroid hormone 
treatment 
weight gain 
weight loss
a b s t r a c t
Objective: Thyroid dysfunction produces characteristic changes in weight and body composition, but 
treatment often results in progressive weight gain.
Methods: This review examines underlying mechanisms, predictors, and implications for patient 
management.
Results: There are significant changes in weight, appetite, and body composition associated with 
underproduction and overproduction of thyroid hormone. The disease states of hypothyroidism and 
hyperthyroidism can be studied in order to document and understand the significant changes in 
body weight that ensue with these conditions. In addition, treatment of these conditions is asso-
ciated with further alterations in body weight. As will be discussed, hypothyroidism is associated 
with mild to modest increases in body weight and accompanying changes in body composition, 
with partial reversal of these alterations with its treatment with thyroid hormone. Ongoing treat-
ment of hypothyroidism tends to be associated with ongoing weight gains. In contrast, hyperthy-
roidism can be accompanied by profound weight loss, with a decrease in fat mass, muscle mass, and 
bone mass, with reversal of the weight loss with restoration of euthyroidism. Specifically, the 
transition to euthyroidism with treatment of hyperthyroidism is accompanied by an increase in fat 
mass, muscle mass, and bone mass. However, resolution of hyperthyroidism typically is associated 
over time with a net increase in body weight that significantly exceeds the nadir seen during 
hyperthyroidism.
Conclusion: Understanding these patterns of weight changes described above is critical for clinicians 
to appreciate so that prior to treatment patients can be counseled about what to expect, and then 
after treatment strategies can be developed to prevent or minimize long-term weight gain after 
restoration of euthyroidism.
© 2025 AACE. Published by Elsevier Inc. This is an open access article under the CC BY license (http:// 
creativecommons.org/licenses/by/4.0/).
Introduction
Thyroid hormone plays a significant role in weight regulation. 1 
However, while thyroid hormone is a recognized regulator of 
weight and body composition, it is only one of many hormonal and 
nonhormonal influences. 2 Moreover, there is redundancy of the 
hormonal systems that control body weight. 3
This review summarizes the significant and complex changes in 
weight, appetite, energy expenditure, and body composition
associated with the development of thyroid disorders. The pro-
found impact that thyroid hormone can have on body weight 
regulation is illustrated by the disease states of hypothyroidism 
and hyperthyroidism, respectively. In addition, the treatment of 
these disorders can also be accompanied by significant changes in 
body weight. Hypothyroidism is associated with modest increases 
in body weight and alterations in body composition. These 
changes are reversed by treatment of hypothyroidism. Hyperthy-
roidism can have an opposite and also significant impact on body 
weight and composition, as it is typically accompanied by pro-
found weight loss. The weight loss is reversed by treatment that 
restores a euthyroid state.
It becomes apparent that reversal of thyroid dysfunction is not 
universally associated with a return to baseline weight but is, in 
fact, more commonly associated with gradual, persistent weight
Abbreviations: BMI, body mass index; TSH, thyroid stimulating hormone.
* Address correspondence to Dr Jacqueline Jonklaas, Division of Endocrinology, 
Georgetown University, 4000 Reservoir Rd, NW, Building D, Suite 230, Washing-
ton, DC 20007.
E-mail address: jonklaaj@georgetown.edu (J. Jonklaas).
Endocrine 
Practice TM
www.endocrinepractice.org
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1530-891X/© 2025 AACE. Published by Elsevier Inc. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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gain. This could be due to the many other factors, both hormonal 
and nonhormonal, also affecting body weight and interacting with 
thyroid status, or it may be linked to suboptimal treatment of 
thyroid disorders. Thyroid disorders are related in a complex 
manner to the development of metabolic syndrome. 4-6 Given the 
prevalence of thyroid disorders, this trend toward weight gain 
could have a significant negative impact on overall population 
health, providing impetus to harness the positive impact of weight 
loss on reducing cardiometabolic risk factors. 7
Weight Changes Associated With Development of Hypothyroidism 
and Its Treatment
The recent Lancet commission on obesity highlights hypothy-
roidism as a secondary cause of obesity. 8 The development of 
hypothyroidism is generally accompanied by decreased energy 
expenditure and decreased thermogenesis, leading to weight gain 
(Table 1). Other factors potentially contributing to weight gain 
accrued during the hypothyroid state include patient fatigu-
ability, 9 reduction of physical activity, 10 and accumulation of gly-
cosaminoglycans. 11,12 A return to the baseline euthyroid body 
weight with treatment of hypothyroidism may not necessarily be 
achieved. This may be hypothesized to be due to such factors as 
continuation of a new set point associated with the higher 
weight, 13 maintenance of the slower metabolism established 
during hypothyroidism, 14 and incomplete normalization of meta-
bolic rate with standard treatment of hypothyroidism. 15,16
Weight Gain With Upper Normal TSH Values and Subclinical 
Hypothyroidism
Several studies implicate even higher thyroid stimulating hor-
mone (TSH) values within the normal range as being associated 
with weight gain. For example, within a population in Denmark, a 
trend for weight gain was described for upper normal TSH values 
that fell within the normal range. 17 Another study in Norway also 
observed that increasing TSH values within the reference range of 
0.3-3.5 mIU/L was associated with weight gain. 18 In data from the 
Framingham Offspring Study, baseline weight increased progres-
sively from the lowest to highest TSH quartile within the range of 
0.5-5.0 mIU/L. 19 Similarly, in a study conducted in Norway, an 
increasing TSH within the reference range was positively associ-
ated with body mass index (BMI) (P for trend 5.0 mIU/L) compared with the rest of the 
sample (9.2% vs 5.7%). 19 A study of patients with subclinical hy-
pothyroidism and a mean TSH of 13 mIU/L showed they had a BMI 
of 25.3 (SD 4.3) kg/m2,compared with a euthyroid control group 
with a TSH of 1.6 mIU/L, which had a BMI of 23.5 (SD 3.5) kg/m2 
(BMI differences Pweight loss or 
decrease in BMI
Initially no change, or return to 
baseline weight and BMI
Initially a return to at least 
baseline weight and BMI
Additional
comments about 
weight changes
Weight loss is due to decreased weight of 
lean body compartment rather than 
decreased fat mass
With ongoing treatment weight 
and BMI may overshoot from 
baseline
With ongoing treatment weight 
and BMI generally overshoot their 
baseline.
With treatment there is an 
increase in both lean mass and fat 
mass.
Weight gain is greater with 
definitive therapies
Mild weight gain is suggested to be approximately 10% or less; moderate weight gain is suggested to be greater than 10%. Abbreviation: BMI = body mass index.
Table 2
Examples of Weight and BMI Decreases Seen in Studies Reporting Treatment of Hypothyroidism
Study (author) Cohort size TSH (mIU/L) Wt (kg) or BMI (kg/m2) P value
Baseline Euthyroid Baseline Euthyroid Amount of decrease
Weight Karmisholt et al 34 12 102 2.2 83.7 79.4 4.3 0.002
Weight Rios-Prego et al 35 17 23.9 3.27 70.9 68.7 2.359 In 1 study, the altered food 
preference was exhibited as increased consumption of carbohy-
drates, but not protein or fat. 59 These changes in food preferences 
may potentially help protect against severe weight loss.
Weight Changes With Treatment of Hyperthyroidism
There is a substantial body of literature demonstrating that 
there is weight gain and other alterations in body composition 
parameters after treatment of hyperthyroidism 58 (Tables 1 and 3). 
Most studies, moreover, suggest a rebound effect such that weight 
is greater than the weight that was believed to be present prior to 
the development of hyperthyroidism. Factors associated with 
greater weight gain (Fig. 1) include higher baseline weight or 
BMI, 50,60 more weight or BMI loss during the period of hyperthy-
roidism, 60 and lower TSH values while hyperthyroid 61,62 (Table 4). 
Regarding lower TSH values, during treatment of subclinical hy-
perthyroidism, it has been reported that increases in BMI are seen 
in those individuals whose initial TSH values were less than 0.1 
mIU/L, but not in those whose initial TSH values were in the 0.1-
0.39 mIU/L range. 62 More weight gain when the hyperthyroidism 
is associated with lower TSH values is also a feature of treating 
overt hyperthyroidism. 61 This finding of increased weight also 
extends to the subset of patients with obesity at the time their 
hyperthyroidism is diagnosed. 50 Many additional factors may 
affect the amount of weight gain, including whether there is an 
intervening period of iatrogenic hypothyroidism. 60,63
An example of the weight gain that occurred with the un-
specified treatment of 17 patients with hyperthyroidism is as fol-
lows. Following restoration of euthyroidism, the mean weight of
Fig. 2. Weight trajectories after treatment of hypothyroidism and hyperthyroidism. Approximate weight trajectories with the development of hypothyroidism or hyperthyroidism 
and then trajectories with their respective treatments are shown over time.
J. Jonklaas Endocrine Practice 32 (2026) 406—415
410
patients increased from 65.5 ± 11.6 kg to 68.4 ± 12.8 kg (increase of 
2.9 ± 3.0 kg, P = .001). There were also accompanying increases in 
BMI from a BMI of 26.4 ± 4.4 kg/m2 to a post-treatment BMI of 
27.6 ± 5 kg/m2 (mean increase 1.2 ± 1.2, P = .001). 35 Another study 
reported a mean weight gain of 3.0 kg (SD 4.3) within a year of 
patients being treated with radioactive iodine therapy. 71 Similar 
occurrence of weight gain is also documented in pediatric patients
with Graves’ disease. 72 Usually, weight gain following treatment of 
hyperthyroidism is greater with definitive treatments such as thy-
roidectomy and radioactive iodine ablation than it is with use of 
antithyroidal agents. 60,68 A study is planned to carefully document 
the weight gain seen following treatment of hyperthyroidism, with 
comparison to a general population control group and estimation of 
the cardiometabolic risks associated with the weight gain. 73 A 2025 
study examined weight changes with treatment of hyperthyroid-
ism, with 84% of participants receiving thionamides and the 
remainder being treated with thyroidectomy or radioactive 
iodine. 55 The mean weight gain following treatment was 
7.4 ± 5.22 kg. Patients were asked to estimate their premorbid 
weight, and based on this, the mean weight loss as a result of hy-
perthyroidism was estimated as 5.3 ± 5.5 kg.
Weight Changes With Specific Therapies for Hyperthyroidism
Definitive therapies for Graves’ disease, in which the hyper-
thyroidism is fully reversed and most patients ultimately need 
levothyroxine treatment for the resultant hypothyroidism, include 
thyroidectomy and radioactive iodine ablation. One study followed 
160 patients with hyperthyroidism, who mostly had Graves’ dis-
ease and were primarily treated with radioactive iodine. Their 
median weight gain after therapy was 5.0 kg at 6 months, 9.0 kg 
after 12 months, and 12 kg after 24 months. 68 A study of 65 pa-
tients with hyperthyroidism compared the weight gain that 
accrued at 1 year after each of 3 therapies for hyperthyroidism. The 
weight gains were means of 5.4 kg (95% CI 3.6 to 7.2 kg) for car-
bimazole treatment, 6.3 kg (95% CI 3.4 to 9.2 kg) for thyroidectomy, 
and 7.4 kg (95% CI 5.2 to 9.6 kg) for radioactive iodine treatment. 
Although the weight gain was not significantly different by treat-
ment group, it was significant at Phormone.
J. Jonklaas Endocrine Practice 32 (2026) 406—415
411
received surgery, radioactive iodine, or methimazole, and those 
who were rendered hypothyroid by their therapy and who 
required levothyroxine treatment were found to have gained more 
weight (10.1 — 10.4 kg), compared with those who were euthyroid 
after their therapy (3.9 - 4.1 kg). 61 Another study in which 
antithyroidal medications, surgery, or radioactive iodine were 
used as therapy confirmed the occurrence of weight gain but 
additionally showed that women, those with higher pretreatment 
BMIs, and those who received definitive therapy with surgery or 
radioactive iodine gained greater amounts of weight 50 (Table 4). A 
further study confirming these findings found that weight gain 
was of a greater magnitude with radioactive iodine treatment 
compared with antithyroid agents. Furthermore, weight gain was 
also associated with any period of TSH elevation and the need for 
thyroid hormone replacement, compared with the group of pa-
tients who did not experience an elevated TSH with need for lev-
othyroxine replacement. The predicted overall weight gain 
associated with development of iatrogenic hypothyroidism was 
1.8 kg in 1 study 63 (Table 4).
Anthropometric and Metabolic Changes With Therapy for 
Hyperthyroidism
Various trends in anthropometric indices have been described 
following treatment of hyperthyroidism (Table 5), which also in-
dicates the measurement technique used. In a group of 75 patients 
treated with radioactive iodine therapy, the increase in BMI at 5 
years was mostly attributable to an increase in lean body mass of 
7.2 kg (P = .0004). 69 However, increases in fat mass also occur with 
hyperthyroidism treatment, as documented in another study. 65 
Fifty patients with Graves’ disease were treated with methimazole 
and had weight and BMI measured at baseline and then at 6 and 12 
months after treatment. Body weight at baseline was 59.02
(±11.11) kg and increased to 64.29 (±9.70) kg at 6 months and then 
to 65.78 (±9.51) kg at 12 months (Penergy balance and weight gain. Thus, a continuation of 
the increased appetite with attendant overeating or increased 
carbohydrate intake is possible. Reduced spontaneous physical 
activity after treatment may further decrease total daily energy 
expenditure.
Comparison of weight trajectories by treatment modality in 
hyperthyroidism using randomized controlled trials are lacking 
and may be difficult to perform due to patient preference being the 
primary driver of selection of treatment modality. Long-term 
cardiometabolic consequences of post-treatment weight gain 
remain underexplored. When the EGRET cohort study is 
completed, the data generated will hopefully shed light on the toll 
of adverse metabolic consequences and cardiovascular risk that is 
seen with the trend for weight gain following hyperthyroidism 
treatment. 73 The propensity for weight gain suggests the impor-
tance of patient education prior to initiating treatment, avoidance 
of iatrogenic hypothyroidism, and other lifestyle measures to avoid 
weight gain above baseline and then continued weight gain 
thereafter following therapy for hyperthyroidism (Fig. 2). 63 Phar-
macologic measures and surgical intervention may also be needed. 
Taken together, this body of evidence also calls for research to fully 
optimize the treatment of hypothyroidism. For the subset of in-
dividuals who require levothyroxine treatment for hypothyroid-
ism after treatment of their hyperthyroidism is completed, it is 
possible that energy expenditure is not fully normalized and is 
lower than in control individuals with native euthyroidism, as 
suggested by the study by Hayashi et al. 66 This raises the issue of 
whether levothyroxine may not serve as fully adequate treatment
for hypothyroidism, whether due to low triiodothyronine levels, 
abnormal thyroxine-to-triiodothyronine ratios, or other factors.
Conclusions
This literature review emphasizes the profound effects of thy-
roid disease on body weight and body composition and justifies 
the classification of thyroid diseases as diseases with a major 
public health impact on obesity, metabolic disease, and cardio-
vascular disease. 76 Although the development of hypothyroidism 
and hyperthyroidism is associated with weight gain and weight 
loss, respectively, the trend that is most unmistakable after 
treatment of these diseases is one of weight gain over time (see 
Box 1). Development of hypothyroidism is generally accompanied 
by a mild to modest weight gain, but treatment of hypothyroidism 
is not universally associated with a reduction in weight. Hyper-
thyroidism, on the other hand, is commonly associated with 
weight loss with decrease in fat mass, muscle mass, and bone 
mass. As treatment is initiated, the change from hyperthyroidism 
to euthyroidism is accompanied by an increase in fat mass, muscle 
mass, and bone mass. However, typically the increase in body 
weight and BMI is believed to rebound significantly above the 
prehyperthyroid baseline. This highlights the need for addressing 
this topic as a research gap in future guidelines for the treatment of 
hyperthyroidism and designing studies that can further explore 
this issue.
Box 1
Summary of key findings
Hypothyroidism
• The excess weight reported with hypothyroidism in 2 studies 
was approximately 3.0-3.7 Kg, compared with a control 
group
• Development of hypothyroidism is associated with increased 
body mass index (BMI) and fat mass and stable or decreased 
muscle mass
• Treatment of overt hypothyroidism is associated with weight 
loss of approximately 0.6-4.3 Kg
• Treatment of overt hypothyroidism is associated with alter-
ation in body composition as follows: decreased BMI of 0.1-
1.7 kg/m2 and loss of lean body mass
Hyperthyroidism
• The range of weight loss reported with the development of 
hyperthyroidism is approximately 2.1 — 7.4 Kg
• Development of hyperthyroidism is associated with 
decreased BMI and fat mass
• Treatment of hyperthyroidism is associated with weight gain 
of approximately 2.7-12 kg
• Predictors of weight gain include lower TSH values and 
higher free T4 at time of hyperthyroidism and a treatment 
modality associated with ultimate need for levothyroxine 
initiation
• Treatment of hyperthyroidism is associated with alteration in 
body composition as follows: increased BMI, increased fat 
mass, and increased muscle mass
J. Jonklaas Endocrine Practice 32 (2026) 406—415
413
Thus, thyroid dysfunction in general and its treatment appear 
to be intimately linked to inexorable weight gain. Current ap-
proaches to the treatment of hypothyroidism do not appear to hold 
the key to limiting or reversing weight gain. Due to the co-occur-
rence of thyroid disease and weight gain, it appears that restoring 
euthyroidism promptly and optimal management of thyroid dis-
eases play a major role in weight management. Potential weight 
gain should be discussed with patients early on, and its manage-
ment addressed. Other avenues including lifestyle modification, 
use of efficacious weight loss medications, and bariatric surgery 77 
may be complementary to optimum management of thyroid dis-
eases to reduce the health impact of obesity and improve quality of 
life.
Disclosure
The authors have no conflicts of interest to disclose.
Acknowledgment
Jacqueline Jonklaas is supported by National Institutes of 
Health Grants 4UH3DE031248 and 5UL1TR001409.
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	The Influence of Thyroid Dysfunction on Body Composition and Weight Trajectory
	Introduction
	Weight Changes Associated With Development of Hypothyroidism and Its Treatment
	Weight Gain With Upper Normal TSH Values and Subclinical Hypothyroidism
	Weight Gain With Development of Overt Hypothyroidism
	Highlights
	Clinical Relevance
	Weight Changes With Treatment of Hypothyroidism
	Possible Mechanisms Explaining Ongoing Weight Gain After Hypothyroidism Treatment
	Weight Changes Associated With Development of Hyperthyroidism and Its Treatment
	Weight Loss With Development of Hyperthyroidism
	Appetite and Food Preference Changes With Hyperthyroidism
	Weight Changes With Treatment of Hyperthyroidism
	Weight Changes With Specific Therapies for Hyperthyroidism
	Anthropometric and Metabolic Changes With Therapy for Hyperthyroidism
	Possible Mechanisms Explaining Ongoing Weight Gain After Hyperthyroidism Treatment
	Conclusions
	Hypothyroidism
	Hyperthyroidism
	Disclosure
	Acknowledgment
	References